Merge branch 'next' into for-linus

4 files changed, 289 insertions, 130 deletions

diff --git a/Documentation/vm/hugetlbpage.txt b/Documentation/vm/hugetlbpage.txt
index 82a7bd1800b2..bc31636973e3 100644
--- a/Documentation/vm/hugetlbpage.txt
+++ b/Documentation/vm/hugetlbpage.txt
@@ -11,23 +11,21 @@ This optimization is more critical now as bigger and bigger physical memories
 (several GBs) are more readily available.
 
 Users can use the huge page support in Linux kernel by either using the mmap
-system call or standard SYSv shared memory system calls (shmget, shmat).
+system call or standard SYSV shared memory system calls (shmget, shmat).
 
 First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
 (present under "File systems") and CONFIG_HUGETLB_PAGE (selected
 automatically when CONFIG_HUGETLBFS is selected) configuration
 options.
 
-The kernel built with huge page support should show the number of configured
-huge pages in the system by running the "cat /proc/meminfo" command.
+The /proc/meminfo file provides information about the total number of
+persistent hugetlb pages in the kernel's huge page pool.  It also displays
+information about the number of free, reserved and surplus huge pages and the
+default huge page size.  The huge page size is needed for generating the
+proper alignment and size of the arguments to system calls that map huge page
+regions.
 
-/proc/meminfo also provides information about the total number of hugetlb
-pages configured in the kernel.  It also displays information about the
-number of free hugetlb pages at any time.  It also displays information about
-the configured huge page size - this is needed for generating the proper
-alignment and size of the arguments to the above system calls.
-
-The output of "cat /proc/meminfo" will have lines like:
+The output of "cat /proc/meminfo" will include lines like:
 
 .....
 HugePages_Total: vvv
@@ -53,59 +51,63 @@ HugePages_Surp  is short for "surplus," and is the number of huge pages in
 /proc/filesystems should also show a filesystem of type "hugetlbfs" configured
 in the kernel.
 
-/proc/sys/vm/nr_hugepages indicates the current number of configured hugetlb
-pages in the kernel.  Super user can dynamically request more (or free some
-pre-configured) huge pages.
-The allocation (or deallocation) of hugetlb pages is possible only if there are
-enough physically contiguous free pages in system (freeing of huge pages is
-possible only if there are enough hugetlb pages free that can be transferred
-back to regular memory pool).
+/proc/sys/vm/nr_hugepages indicates the current number of "persistent" huge
+pages in the kernel's huge page pool.  "Persistent" huge pages will be
+returned to the huge page pool when freed by a task.  A user with root
+privileges can dynamically allocate more or free some persistent huge pages
+by increasing or decreasing the value of 'nr_hugepages'.
 
-Pages that are used as hugetlb pages are reserved inside the kernel and cannot
-be used for other purposes.
+Pages that are used as huge pages are reserved inside the kernel and cannot
+be used for other purposes.  Huge pages cannot be swapped out under
+memory pressure.
 
-Once the kernel with Hugetlb page support is built and running, a user can
-use either the mmap system call or shared memory system calls to start using
-the huge pages.  It is required that the system administrator preallocate
-enough memory for huge page purposes.
+Once a number of huge pages have been pre-allocated to the kernel huge page
+pool, a user with appropriate privilege can use either the mmap system call
+or shared memory system calls to use the huge pages.  See the discussion of
+Using Huge Pages, below.
 
-The administrator can preallocate huge pages on the kernel boot command line by
-specifying the "hugepages=N" parameter, where 'N' = the number of huge pages
-requested.  This is the most reliable method for preallocating huge pages as
-memory has not yet become fragmented.
+The administrator can allocate persistent huge pages on the kernel boot
+command line by specifying the "hugepages=N" parameter, where 'N' = the
+number of huge pages requested.  This is the most reliable method of
+allocating huge pages as memory has not yet become fragmented.
 
-Some platforms support multiple huge page sizes.  To preallocate huge pages
+Some platforms support multiple huge page sizes.  To allocate huge pages
 of a specific size, one must preceed the huge pages boot command parameters
 with a huge page size selection parameter "hugepagesz=<size>".  <size> must
 be specified in bytes with optional scale suffix [kKmMgG].  The default huge
 page size may be selected with the "default_hugepagesz=<size>" boot parameter.
 
-/proc/sys/vm/nr_hugepages indicates the current number of configured [default
-size] hugetlb pages in the kernel.  Super user can dynamically request more
-(or free some pre-configured) huge pages.
-
-Use the following command to dynamically allocate/deallocate default sized
-huge pages:
+When multiple huge page sizes are supported, /proc/sys/vm/nr_hugepages
+indicates the current number of pre-allocated huge pages of the default size.
+Thus, one can use the following command to dynamically allocate/deallocate
+default sized persistent huge pages:
 
         echo 20 > /proc/sys/vm/nr_hugepages
 
-This command will try to configure 20 default sized huge pages in the system.
+This command will try to adjust the number of default sized huge pages in the
+huge page pool to 20, allocating or freeing huge pages, as required.
+
 On a NUMA platform, the kernel will attempt to distribute the huge page pool
-over the all on-line nodes.  These huge pages, allocated when nr_hugepages
-is increased, are called "persistent huge pages".
+over all the set of allowed nodes specified by the NUMA memory policy of the
+task that modifies nr_hugepages.  The default for the allowed nodes--when the
+task has default memory policy--is all on-line nodes with memory.  Allowed
+nodes with insufficient available, contiguous memory for a huge page will be
+silently skipped when allocating persistent huge pages.  See the discussion
+below of the interaction of task memory policy, cpusets and per node attributes
+with the allocation and freeing of persistent huge pages.
 
 The success or failure of huge page allocation depends on the amount of
-physically contiguous memory that is preset in system at the time of the
+physically contiguous memory that is present in system at the time of the
 allocation attempt.  If the kernel is unable to allocate huge pages from
 some nodes in a NUMA system, it will attempt to make up the difference by
 allocating extra pages on other nodes with sufficient available contiguous
 memory, if any.
 
-System administrators may want to put this command in one of the local rc init
-files.  This will enable the kernel to request huge pages early in the boot
-process when the possibility of getting physical contiguous pages is still
-very high.  Administrators can verify the number of huge pages actually
-allocated by checking the sysctl or meminfo.  To check the per node
+System administrators may want to put this command in one of the local rc
+init files.  This will enable the kernel to allocate huge pages early in
+the boot process when the possibility of getting physical contiguous pages
+is still very high.  Administrators can verify the number of huge pages
+actually allocated by checking the sysctl or meminfo.  To check the per node
 distribution of huge pages in a NUMA system, use:
 
         cat /sys/devices/system/node/node*/meminfo | fgrep Huge
@@ -113,45 +115,47 @@ distribution of huge pages in a NUMA system, use:
 /proc/sys/vm/nr_overcommit_hugepages specifies how large the pool of
 huge pages can grow, if more huge pages than /proc/sys/vm/nr_hugepages are
 requested by applications.  Writing any non-zero value into this file
-indicates that the hugetlb subsystem is allowed to try to obtain "surplus"
-huge pages from the buddy allocator, when the normal pool is exhausted. As
-these surplus huge pages go out of use, they are freed back to the buddy
-allocator.
+indicates that the hugetlb subsystem is allowed to try to obtain that
+number of "surplus" huge pages from the kernel's normal page pool, when the
+persistent huge page pool is exhausted. As these surplus huge pages become
+unused, they are freed back to the kernel's normal page pool.
 
-When increasing the huge page pool size via nr_hugepages, any surplus
+When increasing the huge page pool size via nr_hugepages, any existing surplus
 pages will first be promoted to persistent huge pages.  Then, additional
 huge pages will be allocated, if necessary and if possible, to fulfill
-the new huge page pool size.
+the new persistent huge page pool size.
 
-The administrator may shrink the pool of preallocated huge pages for
+The administrator may shrink the pool of persistent huge pages for
 the default huge page size by setting the nr_hugepages sysctl to a
 smaller value.  The kernel will attempt to balance the freeing of huge pages
-across all on-line nodes.  Any free huge pages on the selected nodes will
-be freed back to the buddy allocator.
-
-Caveat: Shrinking the pool via nr_hugepages such that it becomes less
-than the number of huge pages in use will convert the balance to surplus
-huge pages even if it would exceed the overcommit value.  As long as
-this condition holds, however, no more surplus huge pages will be
-allowed on the system until one of the two sysctls are increased
-sufficiently, or the surplus huge pages go out of use and are freed.
+across all nodes in the memory policy of the task modifying nr_hugepages.
+Any free huge pages on the selected nodes will be freed back to the kernel's
+normal page pool.
+
+Caveat: Shrinking the persistent huge page pool via nr_hugepages such that
+it becomes less than the number of huge pages in use will convert the balance
+of the in-use huge pages to surplus huge pages.  This will occur even if
+the number of surplus pages it would exceed the overcommit value.  As long as
+this condition holds--that is, until nr_hugepages+nr_overcommit_hugepages is
+increased sufficiently, or the surplus huge pages go out of use and are freed--
+no more surplus huge pages will be allowed to be allocated.
 
 With support for multiple huge page pools at run-time available, much of
-the huge page userspace interface has been duplicated in sysfs. The above
-information applies to the default huge page size which will be
-controlled by the /proc interfaces for backwards compatibility. The root
-huge page control directory in sysfs is:
+the huge page userspace interface in /proc/sys/vm has been duplicated in sysfs.
+The /proc interfaces discussed above have been retained for backwards
+compatibility. The root huge page control directory in sysfs is:
 
         /sys/kernel/mm/hugepages
 
 For each huge page size supported by the running kernel, a subdirectory
-will exist, of the form
+will exist, of the form:
 
         hugepages-${size}kB
 
 Inside each of these directories, the same set of files will exist:
 
         nr_hugepages
+        nr_hugepages_mempolicy
         nr_overcommit_hugepages
         free_hugepages
         resv_hugepages
@@ -159,6 +163,102 @@ Inside each of these directories, the same set of files will exist:
 
 which function as described above for the default huge page-sized case.
 
+
+Interaction of Task Memory Policy with Huge Page Allocation/Freeing
+
+Whether huge pages are allocated and freed via the /proc interface or
+the /sysfs interface using the nr_hugepages_mempolicy attribute, the NUMA
+nodes from which huge pages are allocated or freed are controlled by the
+NUMA memory policy of the task that modifies the nr_hugepages_mempolicy
+sysctl or attribute.  When the nr_hugepages attribute is used, mempolicy
+is ignored.
+
+The recommended method to allocate or free huge pages to/from the kernel
+huge page pool, using the nr_hugepages example above, is:
+
+    numactl --interleave <node-list> echo 20 \
+                                >/proc/sys/vm/nr_hugepages_mempolicy
+
+or, more succinctly:
+
+    numactl -m <node-list> echo 20 >/proc/sys/vm/nr_hugepages_mempolicy
+
+This will allocate or free abs(20 - nr_hugepages) to or from the nodes
+specified in <node-list>, depending on whether number of persistent huge pages
+is initially less than or greater than 20, respectively.  No huge pages will be
+allocated nor freed on any node not included in the specified <node-list>.
+
+When adjusting the persistent hugepage count via nr_hugepages_mempolicy, any
+memory policy mode--bind, preferred, local or interleave--may be used.  The
+resulting effect on persistent huge page allocation is as follows:
+
+1) Regardless of mempolicy mode [see Documentation/vm/numa_memory_policy.txt],
+   persistent huge pages will be distributed across the node or nodes
+   specified in the mempolicy as if "interleave" had been specified.
+   However, if a node in the policy does not contain sufficient contiguous
+   memory for a huge page, the allocation will not "fallback" to the nearest
+   neighbor node with sufficient contiguous memory.  To do this would cause
+   undesirable imbalance in the distribution of the huge page pool, or
+   possibly, allocation of persistent huge pages on nodes not allowed by
+   the task's memory policy.
+
+2) One or more nodes may be specified with the bind or interleave policy.
+   If more than one node is specified with the preferred policy, only the
+   lowest numeric id will be used.  Local policy will select the node where
+   the task is running at the time the nodes_allowed mask is constructed.
+   For local policy to be deterministic, the task must be bound to a cpu or
+   cpus in a single node.  Otherwise, the task could be migrated to some
+   other node at any time after launch and the resulting node will be
+   indeterminate.  Thus, local policy is not very useful for this purpose.
+   Any of the other mempolicy modes may be used to specify a single node.
+
+3) The nodes allowed mask will be derived from any non-default task mempolicy,
+   whether this policy was set explicitly by the task itself or one of its
+   ancestors, such as numactl.  This means that if the task is invoked from a
+   shell with non-default policy, that policy will be used.  One can specify a
+   node list of "all" with numactl --interleave or --membind [-m] to achieve
+   interleaving over all nodes in the system or cpuset.
+
+4) Any task mempolicy specifed--e.g., using numactl--will be constrained by
+   the resource limits of any cpuset in which the task runs.  Thus, there will
+   be no way for a task with non-default policy running in a cpuset with a
+   subset of the system nodes to allocate huge pages outside the cpuset
+   without first moving to a cpuset that contains all of the desired nodes.
+
+5) Boot-time huge page allocation attempts to distribute the requested number
+   of huge pages over all on-lines nodes with memory.
+
+Per Node Hugepages Attributes
+
+A subset of the contents of the root huge page control directory in sysfs,
+described above, will be replicated under each the system device of each
+NUMA node with memory in:
+
+        /sys/devices/system/node/node[0-9]*/hugepages/
+
+Under this directory, the subdirectory for each supported huge page size
+contains the following attribute files:
+
+        nr_hugepages
+        free_hugepages
+        surplus_hugepages
+
+The free_' and surplus_' attribute files are read-only.  They return the number
+of free and surplus [overcommitted] huge pages, respectively, on the parent
+node.
+
+The nr_hugepages attribute returns the total number of huge pages on the
+specified node.  When this attribute is written, the number of persistent huge
+pages on the parent node will be adjusted to the specified value, if sufficient
+resources exist, regardless of the task's mempolicy or cpuset constraints.
+
+Note that the number of overcommit and reserve pages remain global quantities,
+as we don't know until fault time, when the faulting task's mempolicy is
+applied, from which node the huge page allocation will be attempted.
+
+
+Using Huge Pages
+
 If the user applications are going to request huge pages using mmap system
 call, then it is required that system administrator mount a file system of
 type hugetlbfs:
@@ -206,9 +306,11 @@ map_hugetlb.c.
  * requesting huge pages.
  *
  * For the ia64 architecture, the Linux kernel reserves Region number 4 for
- * huge pages.  That means the addresses starting with 0x800000... will need
- * to be specified.  Specifying a fixed address is not required on ppc64,
- * i386 or x86_64.
+ * huge pages.  That means that if one requires a fixed address, a huge page
+ * aligned address starting with 0x800000... will be required.  If a fixed
+ * address is not required, the kernel will select an address in the proper
+ * range.
+ * Other architectures, such as ppc64, i386 or x86_64 are not so constrained.
  *
  * Note: The default shared memory limit is quite low on many kernels,
  * you may need to increase it via:
@@ -237,14 +339,8 @@ map_hugetlb.c.
 
 #define dprintf(x)  printf(x)
 
-/* Only ia64 requires this */
-#ifdef __ia64__
-#define ADDR (void *)(0x8000000000000000UL)
-#define SHMAT_FLAGS (SHM_RND)
-#else
-#define ADDR (void *)(0x0UL)
+#define ADDR (void *)(0x0UL)    /* let kernel choose address */
 #define SHMAT_FLAGS (0)
-#endif
 
 int main(void)
 {
@@ -302,10 +398,12 @@ int main(void)
  * example, the app is requesting memory of size 256MB that is backed by
  * huge pages.
  *
- * For ia64 architecture, Linux kernel reserves Region number 4 for huge pages.
- * That means the addresses starting with 0x800000... will need to be
- * specified.  Specifying a fixed address is not required on ppc64, i386
- * or x86_64.
+ * For the ia64 architecture, the Linux kernel reserves Region number 4 for
+ * huge pages.  That means that if one requires a fixed address, a huge page
+ * aligned address starting with 0x800000... will be required.  If a fixed
+ * address is not required, the kernel will select an address in the proper
+ * range.
+ * Other architectures, such as ppc64, i386 or x86_64 are not so constrained.
  */
 #include <stdlib.h>
 #include <stdio.h>
@@ -317,14 +415,8 @@ int main(void)
 #define LENGTH (256UL*1024*1024)
 #define PROTECTION (PROT_READ | PROT_WRITE)
 
-/* Only ia64 requires this */
-#ifdef __ia64__
-#define ADDR (void *)(0x8000000000000000UL)
-#define FLAGS (MAP_SHARED | MAP_FIXED)
-#else
-#define ADDR (void *)(0x0UL)
+#define ADDR (void *)(0x0UL)    /* let kernel choose address */
 #define FLAGS (MAP_SHARED)
-#endif
 
 void check_bytes(char *addr)
 {
diff --git a/Documentation/vm/hwpoison.txt b/Documentation/vm/hwpoison.txt
index 3ffadf8da61f..12f9ba20ccb7 100644
--- a/Documentation/vm/hwpoison.txt
+++ b/Documentation/vm/hwpoison.txt
@@ -92,16 +92,62 @@ PR_MCE_KILL_GET
 
 Testing:
 
-madvise(MADV_POISON, ....)
+madvise(MADV_HWPOISON, ....)
         (as root)
         Poison a page in the process for testing
 
 
 hwpoison-inject module through debugfs
-        /sys/debug/hwpoison/corrupt-pfn
 
-Inject hwpoison fault at PFN echoed into this file
+/sys/debug/hwpoison/
 
+corrupt-pfn
+
+Inject hwpoison fault at PFN echoed into this file. This does
+some early filtering to avoid corrupted unintended pages in test suites.
+
+unpoison-pfn
+
+Software-unpoison page at PFN echoed into this file. This
+way a page can be reused again.
+This only works for Linux injected failures, not for real
+memory failures.
+
+Note these injection interfaces are not stable and might change between
+kernel versions
+
+corrupt-filter-dev-major
+corrupt-filter-dev-minor
+
+Only handle memory failures to pages associated with the file system defined
+by block device major/minor.  -1U is the wildcard value.
+This should be only used for testing with artificial injection.
+
+corrupt-filter-memcg
+
+Limit injection to pages owned by memgroup. Specified by inode number
+of the memcg.
+
+Example:
+        mkdir /cgroup/hwpoison
+
+        usemem -m 100 -s 1000 &
+        echo `jobs -p` > /cgroup/hwpoison/tasks
+
+        memcg_ino=$(ls -id /cgroup/hwpoison | cut -f1 -d' ')
+        echo $memcg_ino > /debug/hwpoison/corrupt-filter-memcg
+
+        page-types -p `pidof init`   --hwpoison  # shall do nothing
+        page-types -p `pidof usemem` --hwpoison  # poison its pages
+
+corrupt-filter-flags-mask
+corrupt-filter-flags-value
+
+When specified, only poison pages if ((page_flags & mask) == value).
+This allows stress testing of many kinds of pages. The page_flags
+are the same as in /proc/kpageflags. The flag bits are defined in
+include/linux/kernel-page-flags.h and documented in
+Documentation/vm/pagemap.txt
 
 Architecture specific MCE injector
 
diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt
index 262d8e6793a3..b392e496f816 100644
--- a/Documentation/vm/ksm.txt
+++ b/Documentation/vm/ksm.txt
@@ -16,9 +16,9 @@ by sharing the data common between them.  But it can be useful to any
 application which generates many instances of the same data.
 
 KSM only merges anonymous (private) pages, never pagecache (file) pages.
-KSM's merged pages are at present locked into kernel memory for as long
-as they are shared: so cannot be swapped out like the user pages they
-replace (but swapping KSM pages should follow soon in a later release).
+KSM's merged pages were originally locked into kernel memory, but can now
+be swapped out just like other user pages (but sharing is broken when they
+are swapped back in: ksmd must rediscover their identity and merge again).
 
 KSM only operates on those areas of address space which an application
 has advised to be likely candidates for merging, by using the madvise(2)
@@ -44,20 +44,12 @@ includes unmapped gaps (though working on the intervening mapped areas),
 and might fail with EAGAIN if not enough memory for internal structures.
 
 Applications should be considerate in their use of MADV_MERGEABLE,
-restricting its use to areas likely to benefit.  KSM's scans may use
-a lot of processing power, and its kernel-resident pages are a limited
-resource.  Some installations will disable KSM for these reasons.
+restricting its use to areas likely to benefit.  KSM's scans may use a lot
+of processing power: some installations will disable KSM for that reason.
 
 The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
 readable by all but writable only by root:
 
-max_kernel_pages - set to maximum number of kernel pages that KSM may use
-                   e.g. "echo 100000 > /sys/kernel/mm/ksm/max_kernel_pages"
-                   Value 0 imposes no limit on the kernel pages KSM may use;
-                   but note that any process using MADV_MERGEABLE can cause
-                   KSM to allocate these pages, unswappable until it exits.
-                   Default: quarter of memory (chosen to not pin too much)
-
 pages_to_scan    - how many present pages to scan before ksmd goes to sleep
                    e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan"
                    Default: 100 (chosen for demonstration purposes)
@@ -75,7 +67,7 @@ run              - set 0 to stop ksmd from running but keep merged pages,
 
 The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
 
-pages_shared     - how many shared unswappable kernel pages KSM is using
+pages_shared     - how many shared pages are being used
 pages_sharing    - how many more sites are sharing them i.e. how much saved
 pages_unshared   - how many pages unique but repeatedly checked for merging
 pages_volatile   - how many pages changing too fast to be placed in a tree
@@ -87,4 +79,4 @@ pages_volatile embraces several different kinds of activity, but a high
 proportion there would also indicate poor use of madvise MADV_MERGEABLE.
 
 Izik Eidus,
-Hugh Dickins, 24 Sept 2009
+Hugh Dickins, 17 Nov 2009
diff --git a/Documentation/vm/page-types.c b/Documentation/vm/page-types.c
index ea44ea502da1..66e9358e2144 100644
--- a/Documentation/vm/page-types.c
+++ b/Documentation/vm/page-types.c
@@ -1,11 +1,22 @@
 /*
  * page-types: Tool for querying page flags
  *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; version 2.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should find a copy of v2 of the GNU General Public License somewhere on
+ * your Linux system; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ *
  * Copyright (C) 2009 Intel corporation
  *
  * Authors: Wu Fengguang <fengguang.wu@intel.com>
- *
- * Released under the General Public License (GPL).
  */
 
 #define _LARGEFILE64_SOURCE
@@ -100,7 +111,7 @@
 #define BIT(name)               (1ULL << KPF_##name)
 #define BITS_COMPOUND           (BIT(COMPOUND_HEAD) | BIT(COMPOUND_TAIL))
 
-static char *page_flag_names[] = {
+static const char *page_flag_names[] = {
         [KPF_LOCKED]            = "L:locked",
         [KPF_ERROR]             = "E:error",
         [KPF_REFERENCED]        = "R:referenced",
@@ -173,7 +184,7 @@ static int		kpageflags_fd;
 static int              opt_hwpoison;
 static int              opt_unpoison;
 
-static char             *hwpoison_debug_fs = "/debug/hwpoison";
+static const char       hwpoison_debug_fs[] = "/debug/hwpoison";
 static int              hwpoison_inject_fd;
 static int              hwpoison_forget_fd;
 
@@ -560,7 +571,7 @@ static void walk_pfn(unsigned long voffset,
 {
         uint64_t buf[KPAGEFLAGS_BATCH];
         unsigned long batch;
-        unsigned long pages;
+        long pages;
         unsigned long i;
 
         while (count) {
@@ -673,30 +684,35 @@ static void usage(void)
 
         printf(
 "page-types [options]\n"
-"            -r|--raw                  Raw mode, for kernel developers\n"
-"            -a|--addr    addr-spec    Walk a range of pages\n"
-"            -b|--bits    bits-spec    Walk pages with specified bits\n"
-"            -p|--pid     pid          Walk process address space\n"
+"            -r|--raw                   Raw mode, for kernel developers\n"
+"            -d|--describe flags        Describe flags\n"
+"            -a|--addr    addr-spec     Walk a range of pages\n"
+"            -b|--bits    bits-spec     Walk pages with specified bits\n"
+"            -p|--pid     pid           Walk process address space\n"
 #if 0 /* planned features */
-"            -f|--file    filename     Walk file address space\n"
+"            -f|--file    filename      Walk file address space\n"
 #endif
-"            -l|--list                 Show page details in ranges\n"
-"            -L|--list-each            Show page details one by one\n"
-"            -N|--no-summary           Don't show summay info\n"
-"            -X|--hwpoison             hwpoison pages\n"
-"            -x|--unpoison             unpoison pages\n"
-"            -h|--help                 Show this usage message\n"
+"            -l|--list                  Show page details in ranges\n"
+"            -L|--list-each             Show page details one by one\n"
+"            -N|--no-summary            Don't show summay info\n"
+"            -X|--hwpoison              hwpoison pages\n"
+"            -x|--unpoison              unpoison pages\n"
+"            -h|--help                  Show this usage message\n"
+"flags:\n"
+"            0x10                       bitfield format, e.g.\n"
+"            anon                       bit-name, e.g.\n"
+"            0x10,anon                  comma-separated list, e.g.\n"
 "addr-spec:\n"
-"            N                         one page at offset N (unit: pages)\n"
-"            N+M                       pages range from N to N+M-1\n"
-"            N,M                       pages range from N to M-1\n"
-"            N,                        pages range from N to end\n"
-"            ,M                        pages range from 0 to M-1\n"
+"            N                          one page at offset N (unit: pages)\n"
+"            N+M                        pages range from N to N+M-1\n"
+"            N,M                        pages range from N to M-1\n"
+"            N,                         pages range from N to end\n"
+"            ,M                         pages range from 0 to M-1\n"
 "bits-spec:\n"
-"            bit1,bit2                 (flags & (bit1|bit2)) != 0\n"
-"            bit1,bit2=bit1            (flags & (bit1|bit2)) == bit1\n"
-"            bit1,~bit2                (flags & (bit1|bit2)) == bit1\n"
-"            =bit1,bit2                flags == (bit1|bit2)\n"
+"            bit1,bit2                  (flags & (bit1|bit2)) != 0\n"
+"            bit1,bit2=bit1             (flags & (bit1|bit2)) == bit1\n"
+"            bit1,~bit2                 (flags & (bit1|bit2)) == bit1\n"
+"            =bit1,bit2                 flags == (bit1|bit2)\n"
 "bit-names:\n"
         );
 
@@ -884,13 +900,23 @@ static void parse_bits_mask(const char *optarg)
         add_bits_filter(mask, bits);
 }
 
+static void describe_flags(const char *optarg)
+{
+        uint64_t flags = parse_flag_names(optarg, 0);
+
+        printf("0x%016llx\t%s\t%s\n",
+                (unsigned long long)flags,
+                page_flag_name(flags),
+                page_flag_longname(flags));
+}
 
-static struct option opts[] = {
+static const struct option opts[] = {
         { "raw"       , 0, NULL, 'r' },
         { "pid"       , 1, NULL, 'p' },
         { "file"      , 1, NULL, 'f' },
         { "addr"      , 1, NULL, 'a' },
         { "bits"      , 1, NULL, 'b' },
+        { "describe"  , 1, NULL, 'd' },
         { "list"      , 0, NULL, 'l' },
         { "list-each" , 0, NULL, 'L' },
         { "no-summary", 0, NULL, 'N' },
@@ -907,7 +933,7 @@ int main(int argc, char *argv[])
         page_size = getpagesize();
 
         while ((c = getopt_long(argc, argv,
-                                "rp:f:a:b:lLNXxh", opts, NULL)) != -1) {
+                                "rp:f:a:b:d:lLNXxh", opts, NULL)) != -1) {
                 switch (c) {
                 case 'r':
                         opt_raw = 1;
@@ -924,6 +950,9 @@ int main(int argc, char *argv[])
                 case 'b':
                         parse_bits_mask(optarg);
                         break;
+                case 'd':
+                        describe_flags(optarg);
+                        exit(0);
                 case 'l':
                         opt_list = 1;
                         break;