pagemap: add mmap-exclusive bit for marking pages mapped only here

This patch sets bit 56 in pagemap if this page is mapped only once.  It
allows to detect exclusively used pages without exposing PFN:

present file exclusive state
0       0    0         non-present
1       1    0         file page mapped somewhere else
1       1    1         file page mapped only here
1       0    0         anon non-CoWed page (shared with parent/child)
1       0    1         anon CoWed page (or never forked)

CoWed pages in (MAP_FILE | MAP_PRIVATE) areas are anon in this context.

MMap-exclusive bit doesn't reflect potential page-sharing via swapcache:
page could be mapped once but has several swap-ptes which point to it.
Application could detect that by swap bit in pagemap entry and touch that
pte via /proc/pid/mem to get real information.

See http://lkml.kernel.org/r/CAEVpBa+_RyACkhODZrRvQLs80iy0sqpdrd0AaP_-tgnX3Y9yNQ@mail.gmail.com

Requested by Mark Williamson.

[akpm@linux-foundation.org: fix spello]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: Mark Williamson <mwilliamson@undo-software.com>
Tested-by:  Mark Williamson <mwilliamson@undo-software.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index bc651644b1b2..67c76468a7be 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -949,6 +949,7 @@ struct pagemapread {
 #define PM_PFRAME_BITS          55
 #define PM_PFRAME_MASK          GENMASK_ULL(PM_PFRAME_BITS - 1, 0)
 #define PM_SOFT_DIRTY           BIT_ULL(55)
+#define PM_MMAP_EXCLUSIVE       BIT_ULL(56)
 #define PM_FILE                 BIT_ULL(61)
 #define PM_SWAP                 BIT_ULL(62)
 #define PM_PRESENT              BIT_ULL(63)
@@ -1036,6 +1037,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
 
         if (page && !PageAnon(page))
                 flags |= PM_FILE;
+        if (page && page_mapcount(page) == 1)
+                flags |= PM_MMAP_EXCLUSIVE;
         if (vma->vm_flags & VM_SOFTDIRTY)
                 flags |= PM_SOFT_DIRTY;
 
@@ -1066,6 +1069,11 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
                  * This if-check is just to prepare for future implementation.
                  */
                 if (pmd_present(pmd)) {
+                        struct page *page = pmd_page(pmd);
+
+                        if (page_mapcount(page) == 1)
+                                flags |= PM_MMAP_EXCLUSIVE;
+
                         flags |= PM_PRESENT;
                         if (pm->show_pfn)
                                 frame = pmd_pfn(pmd) +
@@ -1131,6 +1139,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask,
                 if (!PageAnon(page))
                         flags |= PM_FILE;
 
+                if (page_mapcount(page) == 1)
+                        flags |= PM_MMAP_EXCLUSIVE;
+
                 flags |= PM_PRESENT;
                 if (pm->show_pfn)
                         frame = pte_pfn(pte) +
@@ -1163,7 +1174,8 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask,
  * Bits 0-4   swap type if swapped
  * Bits 5-54  swap offset if swapped
  * Bit  55    pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
- * Bits 56-60 zero
+ * Bit  56    page exclusively mapped
+ * Bits 57-60 zero
  * Bit  61    page is file-page or shared-anon
  * Bit  62    page swapped
  * Bit  63    page present