Re: [PATCH v3 2/4] pagemap: add mmap-exclusive bit for marking pages mapped only here

From: Konstantin Khlebnikov <redacted>

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.

Signed-off-by: Konstantin Khlebnikov <redacted>
Link: http://lkml.kernel.org/r/CAEVpBa+_RyACkhODZrRvQLs80iy0sqpdrd0AaP_-tgnX3Y9yNQ@mail.gmail.com

---

v2:
* handle transparent huge pages
* invert bit and rename shared -> exclusive (less confusing name)
---
 Documentation/vm/pagemap.txt |    3 ++-
 fs/proc/task_mmu.c           |   10 ++++++++++
 tools/vm/page-types.c        |   12 ++++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt
index 6bfbc17..3cfbbb3 100644
--- a/Documentation/vm/pagemap.txt
+++ b/Documentation/vm/pagemap.txt

@@ -16,7 +16,8 @@ There are three components to pagemap:
     * 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 exlusively mapped
+    * Bits 57-60 zero
     * Bit  61    page is file-page or shared-anon
     * Bit  62    page swapped
     * Bit  63    page present

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 21bc251..b02e38f 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c

@@ -982,6 +982,7 @@ struct pagemapread {
 #define PM_STATUS2(v2, x)   (__PM_PSHIFT(v2 ? x : PAGE_SHIFT))

 #define __PM_SOFT_DIRTY      (1LL)
+#define __PM_MMAP_EXCLUSIVE  (2LL)
 #define PM_PRESENT          PM_STATUS(4LL)
 #define PM_SWAP             PM_STATUS(2LL)
 #define PM_FILE             PM_STATUS(1LL)

@@ -1074,6 +1075,8 @@ static void pte_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,

        if (page && !PageAnon(page))
                flags |= PM_FILE;
+       if (page && page_mapcount(page) == 1)
+               flags2 |= __PM_MMAP_EXCLUSIVE;
        if ((vma->vm_flags & VM_SOFTDIRTY))
                flags2 |= __PM_SOFT_DIRTY;

@@ -1119,6 +1122,13 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
                else
                        pmd_flags2 = 0;

+               if (pmd_present(*pmd)) {
+                       struct page *page = pmd_page(*pmd);
+
+                       if (page_mapcount(page) == 1)
+                               pmd_flags2 |= __PM_MMAP_EXCLUSIVE;
+               }
+
                for (; addr != end; addr += PAGE_SIZE) {
                        unsigned long offset;
                        pagemap_entry_t pme;

diff --git a/tools/vm/page-types.c b/tools/vm/page-types.c
index 8bdf16b..3a9f193 100644
--- a/tools/vm/page-types.c
+++ b/tools/vm/page-types.c

@@ -70,9 +70,12 @@
 #define PM_PFRAME(x)        ((x) & PM_PFRAME_MASK)

 #define __PM_SOFT_DIRTY      (1LL)
+#define __PM_MMAP_EXCLUSIVE  (2LL)
 #define PM_PRESENT          PM_STATUS(4LL)
 #define PM_SWAP             PM_STATUS(2LL)
+#define PM_FILE             PM_STATUS(1LL)
 #define PM_SOFT_DIRTY       __PM_PSHIFT(__PM_SOFT_DIRTY)
+#define PM_MMAP_EXCLUSIVE   __PM_PSHIFT(__PM_MMAP_EXCLUSIVE)


 /*

@@ -100,6 +103,8 @@
 #define KPF_SLOB_FREE          49
 #define KPF_SLUB_FROZEN                50
 #define KPF_SLUB_DEBUG         51
+#define KPF_FILE               62
+#define KPF_MMAP_EXCLUSIVE     63

 #define KPF_ALL_BITS           ((uint64_t)~0ULL)
 #define KPF_HACKERS_BITS       (0xffffULL << 32)

@@ -149,6 +154,9 @@ static const char * const page_flag_names[] = {
        [KPF_SLOB_FREE]         = "P:slob_free",
        [KPF_SLUB_FROZEN]       = "A:slub_frozen",
        [KPF_SLUB_DEBUG]        = "E:slub_debug",
+
+       [KPF_FILE]              = "F:file",
+       [KPF_MMAP_EXCLUSIVE]    = "1:mmap_exclusive",
 };

@@ -452,6 +460,10 @@ static uint64_t expand_overloaded_flags(uint64_t flags, uint64_t pme)

        if (pme & PM_SOFT_DIRTY)
                flags |= BIT(SOFTDIRTY);
+       if (pme & PM_FILE)
+               flags |= BIT(FILE);
+       if (pme & PM_MMAP_EXCLUSIVE)
+               flags |= BIT(MMAP_EXCLUSIVE);

        return flags;
 }