From 77bb499bb60f4b79cca7d139c8041662860fcf87 Mon Sep 17 00:00:00 2001 From: Konstantin Khlebnikov Date: Tue, 8 Sep 2015 15:00:10 -0700 Subject: 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 Reviewed-by: Mark Williamson Tested-by: Mark Williamson Reviewed-by: Naoya Horiguchi Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/vm/pagemap.txt | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'Documentation/vm/pagemap.txt') diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt index 6bfbc172cdb9..56faec0f73f7 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 exclusively mapped + * Bits 57-60 zero * Bit 61 page is file-page or shared-anon * Bit 62 page swapped * Bit 63 page present -- cgit v1.2.1 From 83b4b0bb635eee2b8e075062e4e008d1bc110ed7 Mon Sep 17 00:00:00 2001 From: Konstantin Khlebnikov Date: Tue, 8 Sep 2015 15:00:13 -0700 Subject: pagemap: update documentation Notes about recent changes. [akpm@linux-foundation.org: various tweaks] Signed-off-by: Konstantin Khlebnikov Cc: Mark Williamson Cc: Naoya Horiguchi Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/vm/pagemap.txt | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) (limited to 'Documentation/vm/pagemap.txt') diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt index 56faec0f73f7..3cd38438242a 100644 --- a/Documentation/vm/pagemap.txt +++ b/Documentation/vm/pagemap.txt @@ -16,12 +16,17 @@ 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) - * Bit 56 page exclusively mapped + * Bit 56 page exclusively mapped (since 4.2) * Bits 57-60 zero - * Bit 61 page is file-page or shared-anon + * Bit 61 page is file-page or shared-anon (since 3.5) * Bit 62 page swapped * Bit 63 page present + Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs. + In 4.0 and 4.1 opens by unprivileged fail with -EPERM. Starting from + 4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN. + Reason: information about PFNs helps in exploiting Rowhammer vulnerability. + If the page is not present but in swap, then the PFN contains an encoding of the swap file number and the page's offset into the swap. Unmapped pages return a null PFN. This allows determining @@ -160,3 +165,8 @@ Other notes: Reading from any of the files will return -EINVAL if you are not starting the read on an 8-byte boundary (e.g., if you sought an odd number of bytes into the file), or if the size of the read is not a multiple of 8 bytes. + +Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is +always 12 at most architectures). Since Linux 3.11 their meaning changes +after first clear of soft-dirty bits. Since Linux 4.2 they are used for +flags unconditionally. -- cgit v1.2.1