5 files changed, 622 insertions, 0 deletions

diff --git a/Documentation/vm/balance b/Documentation/vm/balance
new file mode 100644
index 000000000000..bd3d31bc4915
--- /dev/null
+++ b/Documentation/vm/balance
@@ -0,0 +1,93 @@
+Started Jan 2000 by Kanoj Sarcar <kanoj@sgi.com>
+
+Memory balancing is needed for non __GFP_WAIT as well as for non
+__GFP_IO allocations.
+
+There are two reasons to be requesting non __GFP_WAIT allocations:
+the caller can not sleep (typically intr context), or does not want
+to incur cost overheads of page stealing and possible swap io for
+whatever reasons.
+
+__GFP_IO allocation requests are made to prevent file system deadlocks.
+
+In the absence of non sleepable allocation requests, it seems detrimental
+to be doing balancing. Page reclamation can be kicked off lazily, that
+is, only when needed (aka zone free memory is 0), instead of making it
+a proactive process.
+
+That being said, the kernel should try to fulfill requests for direct
+mapped pages from the direct mapped pool, instead of falling back on
+the dma pool, so as to keep the dma pool filled for dma requests (atomic
+or not). A similar argument applies to highmem and direct mapped pages.
+OTOH, if there is a lot of free dma pages, it is preferable to satisfy
+regular memory requests by allocating one from the dma pool, instead
+of incurring the overhead of regular zone balancing.
+
+In 2.2, memory balancing/page reclamation would kick off only when the
+_total_ number of free pages fell below 1/64 th of total memory. With the
+right ratio of dma and regular memory, it is quite possible that balancing
+would not be done even when the dma zone was completely empty. 2.2 has
+been running production machines of varying memory sizes, and seems to be
+doing fine even with the presence of this problem. In 2.3, due to
+HIGHMEM, this problem is aggravated.
+
+In 2.3, zone balancing can be done in one of two ways: depending on the
+zone size (and possibly of the size of lower class zones), we can decide
+at init time how many free pages we should aim for while balancing any
+zone. The good part is, while balancing, we do not need to look at sizes
+of lower class zones, the bad part is, we might do too frequent balancing
+due to ignoring possibly lower usage in the lower class zones. Also,
+with a slight change in the allocation routine, it is possible to reduce
+the memclass() macro to be a simple equality.
+
+Another possible solution is that we balance only when the free memory
+of a zone _and_ all its lower class zones falls below 1/64th of the
+total memory in the zone and its lower class zones. This fixes the 2.2
+balancing problem, and stays as close to 2.2 behavior as possible. Also,
+the balancing algorithm works the same way on the various architectures,
+which have different numbers and types of zones. If we wanted to get
+fancy, we could assign different weights to free pages in different
+zones in the future.
+
+Note that if the size of the regular zone is huge compared to dma zone,
+it becomes less significant to consider the free dma pages while
+deciding whether to balance the regular zone. The first solution
+becomes more attractive then.
+
+The appended patch implements the second solution. It also "fixes" two
+problems: first, kswapd is woken up as in 2.2 on low memory conditions
+for non-sleepable allocations. Second, the HIGHMEM zone is also balanced,
+so as to give a fighting chance for replace_with_highmem() to get a
+HIGHMEM page, as well as to ensure that HIGHMEM allocations do not
+fall back into regular zone. This also makes sure that HIGHMEM pages
+are not leaked (for example, in situations where a HIGHMEM page is in 
+the swapcache but is not being used by anyone)
+
+kswapd also needs to know about the zones it should balance. kswapd is
+primarily needed in a situation where balancing can not be done, 
+probably because all allocation requests are coming from intr context
+and all process contexts are sleeping. For 2.3, kswapd does not really
+need to balance the highmem zone, since intr context does not request
+highmem pages. kswapd looks at the zone_wake_kswapd field in the zone
+structure to decide whether a zone needs balancing.
+
+Page stealing from process memory and shm is done if stealing the page would
+alleviate memory pressure on any zone in the page's node that has fallen below
+its watermark.
+
+pages_min/pages_low/pages_high/low_on_memory/zone_wake_kswapd: These are 
+per-zone fields, used to determine when a zone needs to be balanced. When
+the number of pages falls below pages_min, the hysteric field low_on_memory
+gets set. This stays set till the number of free pages becomes pages_high.
+When low_on_memory is set, page allocation requests will try to free some
+pages in the zone (providing GFP_WAIT is set in the request). Orthogonal
+to this, is the decision to poke kswapd to free some zone pages. That
+decision is not hysteresis based, and is done when the number of free
+pages is below pages_low; in which case zone_wake_kswapd is also set.
+
+
+(Good) Ideas that I have heard:
+1. Dynamic experience should influence balancing: number of failed requests
+for a zone can be tracked and fed into the balancing scheme (jalvo@mbay.net)
+2. Implement a replace_with_highmem()-like replace_with_regular() to preserve
+dma pages. (lkd@tantalophile.demon.co.uk)
diff --git a/Documentation/vm/hugetlbpage.txt b/Documentation/vm/hugetlbpage.txt
new file mode 100644
index 000000000000..1b9bcd1fe98b
--- /dev/null
+++ b/Documentation/vm/hugetlbpage.txt
@@ -0,0 +1,284 @@
+
+The intent of this file is to give a brief summary of hugetlbpage support in
+the Linux kernel.  This support is built on top of multiple page size support
+that is provided by most modern architectures.  For example, i386
+architecture supports 4K and 4M (2M in PAE mode) page sizes, ia64
+architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
+256M and ppc64 supports 4K and 16M.  A TLB is a cache of virtual-to-physical
+translations.  Typically this is a very scarce resource on processor.
+Operating systems try to make best use of limited number of TLB resources.
+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).
+
+First the Linux kernel needs to be built with CONFIG_HUGETLB_PAGE (present
+under Processor types and feature)  and CONFIG_HUGETLBFS (present under file
+system option on config menu) config options.
+
+The kernel built with hugepage support should show the number of configured
+hugepages in the system by running the "cat /proc/meminfo" command.  
+
+/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 hugepage 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 output like:
+
+.....
+HugePages_Total: xxx
+HugePages_Free:  yyy
+Hugepagesize:    zzz KB
+
+/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) hugepages. 
+The allocation( or deallocation) of hugetlb pages is posible only if there are
+enough physically contiguous free pages in system (freeing of hugepages is
+possible only if there are enough hugetlb pages free that can be transfered 
+back to regular memory pool).
+
+Pages that are used as hugetlb pages are reserved inside the kernel and can
+not be used for other purposes. 
+
+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.  
+
+Use the following command to dynamically allocate/deallocate hugepages:
+
+        echo 20 > /proc/sys/vm/nr_hugepages
+
+This command will try to configure 20 hugepages in the system.  The success
+or failure of allocation depends on the amount of physically contiguous
+memory that is preset in system at this time.  System administrators may want
+to put this command in one of the local rc init file.  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).
+
+If the user applications are going to request hugepages using mmap system
+call, then it is required that system administrator mount a file system of
+type hugetlbfs:
+
+        mount none /mnt/huge -t hugetlbfs <uid=value> <gid=value> <mode=value>
+                 <size=value> <nr_inodes=value>
+
+This command mounts a (pseudo) filesystem of type hugetlbfs on the directory
+/mnt/huge.  Any files created on /mnt/huge uses hugepages.  The uid and gid
+options sets the owner and group of the root of the file system.  By default
+the uid and gid of the current process are taken.  The mode option sets the
+mode of root of file system to value & 0777.  This value is given in octal.
+By default the value 0755 is picked. The size option sets the maximum value of
+memory (huge pages) allowed for that filesystem (/mnt/huge). The size is
+rounded down to HPAGE_SIZE.  The option nr_inode sets the maximum number of
+inodes that /mnt/huge can use.  If the size or nr_inode options are not
+provided on command line then no limits are set.  For size and nr_inodes
+options, you can use [G|g]/[M|m]/[K|k] to represent giga/mega/kilo. For 
+example, size=2K has the same meaning as size=2048. An example is given at 
+the end of this document. 
+
+read and write system calls are not supported on files that reside on hugetlb
+file systems.
+
+A regular chown, chgrp and chmod commands (with right permissions) could be
+used to change the file attributes on hugetlbfs.
+
+Also, it is important to note that no such mount command is required if the
+applications are going to use only shmat/shmget system calls.  Users who
+wish to use hugetlb page via shared memory segment should be a member of
+a supplementary group and system admin needs to configure that gid into
+/proc/sys/vm/hugetlb_shm_group.  It is possible for same or different
+applications to use any combination of mmaps and shm* calls.  Though the
+mount of filesystem will be required for using mmaps.
+
+*******************************************************************
+
+/*
+ * Example of using hugepage memory in a user application using Sys V shared
+ * memory system calls.  In this example the app is requesting 256MB of
+ * memory that is backed by huge pages.  The application uses the flag
+ * SHM_HUGETLB in the shmget system call to inform the kernel that it is
+ * requesting hugepages.
+ *
+ * For the ia64 architecture, the Linux kernel reserves Region number 4 for
+ * hugepages.  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.
+ *
+ * Note: The default shared memory limit is quite low on many kernels,
+ * you may need to increase it via:
+ *
+ * echo 268435456 > /proc/sys/kernel/shmmax
+ *
+ * This will increase the maximum size per shared memory segment to 256MB.
+ * The other limit that you will hit eventually is shmall which is the
+ * total amount of shared memory in pages. To set it to 16GB on a system
+ * with a 4kB pagesize do:
+ *
+ * echo 4194304 > /proc/sys/kernel/shmall
+ */
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/ipc.h>
+#include <sys/shm.h>
+#include <sys/mman.h>
+
+#ifndef SHM_HUGETLB
+#define SHM_HUGETLB 04000
+#endif
+
+#define LENGTH (256UL*1024*1024)
+
+#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 SHMAT_FLAGS (0)
+#endif
+
+int main(void)
+{
+        int shmid;
+        unsigned long i;
+        char *shmaddr;
+
+        if ((shmid = shmget(2, LENGTH,
+                            SHM_HUGETLB | IPC_CREAT | SHM_R | SHM_W)) < 0) {
+                perror("shmget");
+                exit(1);
+        }
+        printf("shmid: 0x%x\n", shmid);
+
+        shmaddr = shmat(shmid, ADDR, SHMAT_FLAGS);
+        if (shmaddr == (char *)-1) {
+                perror("Shared memory attach failure");
+                shmctl(shmid, IPC_RMID, NULL);
+                exit(2);
+        }
+        printf("shmaddr: %p\n", shmaddr);
+
+        dprintf("Starting the writes:\n");
+        for (i = 0; i < LENGTH; i++) {
+                shmaddr[i] = (char)(i);
+                if (!(i % (1024 * 1024)))
+                        dprintf(".");
+        }
+        dprintf("\n");
+
+        dprintf("Starting the Check...");
+        for (i = 0; i < LENGTH; i++)
+                if (shmaddr[i] != (char)i)
+                        printf("\nIndex %lu mismatched\n", i);
+        dprintf("Done.\n");
+
+        if (shmdt((const void *)shmaddr) != 0) {
+                perror("Detach failure");
+                shmctl(shmid, IPC_RMID, NULL);
+                exit(3);
+        }
+
+        shmctl(shmid, IPC_RMID, NULL);
+
+        return 0;
+}
+
+*******************************************************************
+
+/*
+ * Example of using hugepage memory in a user application using the mmap
+ * system call.  Before running this application, make sure that the
+ * administrator has mounted the hugetlbfs filesystem (on some directory
+ * like /mnt) using the command mount -t hugetlbfs nodev /mnt. In this
+ * 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 hugepages.
+ * 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.
+ */
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+
+#define FILE_NAME "/mnt/hugepagefile"
+#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 FLAGS (MAP_SHARED)
+#endif
+
+void check_bytes(char *addr)
+{
+        printf("First hex is %x\n", *((unsigned int *)addr));
+}
+
+void write_bytes(char *addr)
+{
+        unsigned long i;
+
+        for (i = 0; i < LENGTH; i++)
+                *(addr + i) = (char)i;
+}
+
+void read_bytes(char *addr)
+{
+        unsigned long i;
+
+        check_bytes(addr);
+        for (i = 0; i < LENGTH; i++)
+                if (*(addr + i) != (char)i) {
+                        printf("Mismatch at %lu\n", i);
+                        break;
+                }
+}
+
+int main(void)
+{
+        void *addr;
+        int fd;
+
+        fd = open(FILE_NAME, O_CREAT | O_RDWR, 0755);
+        if (fd < 0) {
+                perror("Open failed");
+                exit(1);
+        }
+
+        addr = mmap(ADDR, LENGTH, PROTECTION, FLAGS, fd, 0);
+        if (addr == MAP_FAILED) {
+                perror("mmap");
+                unlink(FILE_NAME);
+                exit(1);
+        }
+
+        printf("Returned address is %p\n", addr);
+        check_bytes(addr);
+        write_bytes(addr);
+        read_bytes(addr);
+
+        munmap(addr, LENGTH);
+        close(fd);
+        unlink(FILE_NAME);
+
+        return 0;
+}
diff --git a/Documentation/vm/locking b/Documentation/vm/locking
new file mode 100644
index 000000000000..c3ef09ae3bb1
--- /dev/null
+++ b/Documentation/vm/locking
@@ -0,0 +1,131 @@
+Started Oct 1999 by Kanoj Sarcar <kanojsarcar@yahoo.com>
+
+The intent of this file is to have an uptodate, running commentary 
+from different people about how locking and synchronization is done 
+in the Linux vm code.
+
+page_table_lock & mmap_sem
+--------------------------------------
+
+Page stealers pick processes out of the process pool and scan for 
+the best process to steal pages from. To guarantee the existence 
+of the victim mm, a mm_count inc and a mmdrop are done in swap_out().
+Page stealers hold kernel_lock to protect against a bunch of races.
+The vma list of the victim mm is also scanned by the stealer, 
+and the page_table_lock is used to preserve list sanity against the
+process adding/deleting to the list. This also guarantees existence
+of the vma. Vma existence is not guaranteed once try_to_swap_out() 
+drops the page_table_lock. To guarantee the existence of the underlying 
+file structure, a get_file is done before the swapout() method is 
+invoked. The page passed into swapout() is guaranteed not to be reused
+for a different purpose because the page reference count due to being
+present in the user's pte is not released till after swapout() returns.
+
+Any code that modifies the vmlist, or the vm_start/vm_end/
+vm_flags:VM_LOCKED/vm_next of any vma *in the list* must prevent 
+kswapd from looking at the chain.
+
+The rules are:
+1. To scan the vmlist (look but don't touch) you must hold the
+   mmap_sem with read bias, i.e. down_read(&mm->mmap_sem)
+2. To modify the vmlist you need to hold the mmap_sem with
+   read&write bias, i.e. down_write(&mm->mmap_sem)  *AND*
+   you need to take the page_table_lock.
+3. The swapper takes _just_ the page_table_lock, this is done
+   because the mmap_sem can be an extremely long lived lock
+   and the swapper just cannot sleep on that.
+4. The exception to this rule is expand_stack, which just
+   takes the read lock and the page_table_lock, this is ok
+   because it doesn't really modify fields anybody relies on.
+5. You must be able to guarantee that while holding page_table_lock
+   or page_table_lock of mm A, you will not try to get either lock
+   for mm B.
+
+The caveats are:
+1. find_vma() makes use of, and updates, the mmap_cache pointer hint.
+The update of mmap_cache is racy (page stealer can race with other code
+that invokes find_vma with mmap_sem held), but that is okay, since it 
+is a hint. This can be fixed, if desired, by having find_vma grab the
+page_table_lock.
+
+
+Code that add/delete elements from the vmlist chain are
+1. callers of insert_vm_struct
+2. callers of merge_segments
+3. callers of avl_remove
+
+Code that changes vm_start/vm_end/vm_flags:VM_LOCKED of vma's on
+the list:
+1. expand_stack
+2. mprotect
+3. mlock
+4. mremap
+
+It is advisable that changes to vm_start/vm_end be protected, although 
+in some cases it is not really needed. Eg, vm_start is modified by 
+expand_stack(), it is hard to come up with a destructive scenario without 
+having the vmlist protection in this case.
+
+The page_table_lock nests with the inode i_mmap_lock and the kmem cache
+c_spinlock spinlocks.  This is okay, since the kmem code asks for pages after
+dropping c_spinlock.  The page_table_lock also nests with pagecache_lock and
+pagemap_lru_lock spinlocks, and no code asks for memory with these locks
+held.
+
+The page_table_lock is grabbed while holding the kernel_lock spinning monitor.
+
+The page_table_lock is a spin lock.
+
+Note: PTL can also be used to guarantee that no new clones using the
+mm start up ... this is a loose form of stability on mm_users. For
+example, it is used in copy_mm to protect against a racing tlb_gather_mmu
+single address space optimization, so that the zap_page_range (from
+vmtruncate) does not lose sending ipi's to cloned threads that might 
+be spawned underneath it and go to user mode to drag in pte's into tlbs.
+
+swap_list_lock/swap_device_lock
+-------------------------------
+The swap devices are chained in priority order from the "swap_list" header. 
+The "swap_list" is used for the round-robin swaphandle allocation strategy.
+The #free swaphandles is maintained in "nr_swap_pages". These two together
+are protected by the swap_list_lock. 
+
+The swap_device_lock, which is per swap device, protects the reference 
+counts on the corresponding swaphandles, maintained in the "swap_map"
+array, and the "highest_bit" and "lowest_bit" fields.
+
+Both of these are spinlocks, and are never acquired from intr level. The
+locking hierarchy is swap_list_lock -> swap_device_lock.
+
+To prevent races between swap space deletion or async readahead swapins
+deciding whether a swap handle is being used, ie worthy of being read in
+from disk, and an unmap -> swap_free making the handle unused, the swap
+delete and readahead code grabs a temp reference on the swaphandle to
+prevent warning messages from swap_duplicate <- read_swap_cache_async.
+
+Swap cache locking
+------------------
+Pages are added into the swap cache with kernel_lock held, to make sure
+that multiple pages are not being added (and hence lost) by associating
+all of them with the same swaphandle.
+
+Pages are guaranteed not to be removed from the scache if the page is 
+"shared": ie, other processes hold reference on the page or the associated 
+swap handle. The only code that does not follow this rule is shrink_mmap,
+which deletes pages from the swap cache if no process has a reference on 
+the page (multiple processes might have references on the corresponding
+swap handle though). lookup_swap_cache() races with shrink_mmap, when
+establishing a reference on a scache page, so, it must check whether the
+page it located is still in the swapcache, or shrink_mmap deleted it.
+(This race is due to the fact that shrink_mmap looks at the page ref
+count with pagecache_lock, but then drops pagecache_lock before deleting
+the page from the scache).
+
+do_wp_page and do_swap_page have MP races in them while trying to figure
+out whether a page is "shared", by looking at the page_count + swap_count.
+To preserve the sum of the counts, the page lock _must_ be acquired before
+calling is_page_shared (else processes might switch their swap_count refs
+to the page count refs, after the page count ref has been snapshotted).
+
+Swap device deletion code currently breaks all the scache assumptions,
+since it grabs neither mmap_sem nor page_table_lock.
diff --git a/Documentation/vm/numa b/Documentation/vm/numa
new file mode 100644
index 000000000000..4b8db1bd3b78
--- /dev/null
+++ b/Documentation/vm/numa
@@ -0,0 +1,41 @@
+Started Nov 1999 by Kanoj Sarcar <kanoj@sgi.com>
+
+The intent of this file is to have an uptodate, running commentary 
+from different people about NUMA specific code in the Linux vm.
+
+What is NUMA? It is an architecture where the memory access times
+for different regions of memory from a given processor varies
+according to the "distance" of the memory region from the processor.
+Each region of memory to which access times are the same from any 
+cpu, is called a node. On such architectures, it is beneficial if
+the kernel tries to minimize inter node communications. Schemes
+for this range from kernel text and read-only data replication
+across nodes, and trying to house all the data structures that
+key components of the kernel need on memory on that node.
+
+Currently, all the numa support is to provide efficient handling
+of widely discontiguous physical memory, so architectures which 
+are not NUMA but can have huge holes in the physical address space
+can use the same code. All this code is bracketed by CONFIG_DISCONTIGMEM.
+
+The initial port includes NUMAizing the bootmem allocator code by
+encapsulating all the pieces of information into a bootmem_data_t
+structure. Node specific calls have been added to the allocator. 
+In theory, any platform which uses the bootmem allocator should 
+be able to to put the bootmem and mem_map data structures anywhere
+it deems best.
+
+Each node's page allocation data structures have also been encapsulated
+into a pg_data_t. The bootmem_data_t is just one part of this. To 
+make the code look uniform between NUMA and regular UMA platforms, 
+UMA platforms have a statically allocated pg_data_t too (contig_page_data).
+For the sake of uniformity, the function num_online_nodes() is also defined
+for all platforms. As we run benchmarks, we might decide to NUMAize 
+more variables like low_on_memory, nr_free_pages etc into the pg_data_t.
+
+The NUMA aware page allocation code currently tries to allocate pages 
+from different nodes in a round robin manner.  This will be changed to 
+do concentratic circle search, starting from current node, once the 
+NUMA port achieves more maturity. The call alloc_pages_node has been 
+added, so that drivers can make the call and not worry about whether 
+it is running on a NUMA or UMA platform.
diff --git a/Documentation/vm/overcommit-accounting b/Documentation/vm/overcommit-accounting
new file mode 100644
index 000000000000..21c7b1f8f32b
--- /dev/null
+++ b/Documentation/vm/overcommit-accounting
@@ -0,0 +1,73 @@
+The Linux kernel supports the following overcommit handling modes
+
+0       -       Heuristic overcommit handling. Obvious overcommits of
+                address space are refused. Used for a typical system. It
+                ensures a seriously wild allocation fails while allowing
+                overcommit to reduce swap usage.  root is allowed to 
+                allocate slighly more memory in this mode. This is the 
+                default.
+
+1       -       Always overcommit. Appropriate for some scientific
+                applications.
+
+2       -       Don't overcommit. The total address space commit
+                for the system is not permitted to exceed swap + a
+                configurable percentage (default is 50) of physical RAM.
+                Depending on the percentage you use, in most situations
+                this means a process will not be killed while accessing
+                pages but will receive errors on memory allocation as
+                appropriate.
+
+The overcommit policy is set via the sysctl `vm.overcommit_memory'.
+
+The overcommit percentage is set via `vm.overcommit_ratio'.
+
+The current overcommit limit and amount committed are viewable in
+/proc/meminfo as CommitLimit and Committed_AS respectively.
+
+Gotchas
+-------
+
+The C language stack growth does an implicit mremap. If you want absolute
+guarantees and run close to the edge you MUST mmap your stack for the 
+largest size you think you will need. For typical stack usage this does
+not matter much but it's a corner case if you really really care
+
+In mode 2 the MAP_NORESERVE flag is ignored. 
+
+
+How It Works
+------------
+
+The overcommit is based on the following rules
+
+For a file backed map
+        SHARED or READ-only     -       0 cost (the file is the map not swap)
+        PRIVATE WRITABLE        -       size of mapping per instance
+
+For an anonymous or /dev/zero map
+        SHARED                  -       size of mapping
+        PRIVATE READ-only       -       0 cost (but of little use)
+        PRIVATE WRITABLE        -       size of mapping per instance
+
+Additional accounting
+        Pages made writable copies by mmap
+        shmfs memory drawn from the same pool
+
+Status
+------
+
+o       We account mmap memory mappings
+o       We account mprotect changes in commit
+o       We account mremap changes in size
+o       We account brk
+o       We account munmap
+o       We report the commit status in /proc
+o       Account and check on fork
+o       Review stack handling/building on exec
+o       SHMfs accounting
+o       Implement actual limit enforcement
+
+To Do
+-----
+o       Account ptrace pages (this is hard)