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authorVinayak Menon <vinmenon@codeaurora.org>2019-11-15 17:35:00 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2019-11-15 18:34:00 -0800
commit5df373e95689b9519b8557da7c5bd0db0856d776 (patch)
treea6a71b055740431d720c0af41c45a98674db3e42 /mm/debug.c
parent2c91f8fc6c999fe10185d8ad99fda1759f662f70 (diff)
downloadlinux-5df373e95689b9519b8557da7c5bd0db0856d776.tar.bz2
mm/page_io.c: do not free shared swap slots
The following race is observed due to which a processes faulting on a swap entry, finds the page neither in swapcache nor swap. This causes zram to give a zero filled page that gets mapped to the process, resulting in a user space crash later. Consider parent and child processes Pa and Pb sharing the same swap slot with swap_count 2. Swap is on zram with SWP_SYNCHRONOUS_IO set. Virtual address 'VA' of Pa and Pb points to the shared swap entry. Pa Pb fault on VA fault on VA do_swap_page do_swap_page lookup_swap_cache fails lookup_swap_cache fails Pb scheduled out swapin_readahead (deletes zram entry) swap_free (makes swap_count 1) Pb scheduled in swap_readpage (swap_count == 1) Takes SWP_SYNCHRONOUS_IO path zram enrty absent zram gives a zero filled page Fix this by making sure that swap slot is freed only when swap count drops down to one. Link: http://lkml.kernel.org/r/1571743294-14285-1-git-send-email-vinmenon@codeaurora.org Fixes: aa8d22a11da9 ("mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference") Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org> Suggested-by: Minchan Kim <minchan@google.com> Acked-by: Minchan Kim <minchan@kernel.org> Cc: Michal Hocko <mhocko@suse.com> Cc: Hugh Dickins <hughd@google.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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