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    Subject[PATCH AUTOSEL 4.14 117/127] mm/swap_state: fix a data race in swapin_nr_pages
    Date
    From: Qian Cai <cai@lca.pw>

    [ Upstream commit d6c1f098f2a7ba62627c9bc17cda28f534ef9e4a ]

    "prev_offset" is a static variable in swapin_nr_pages() that can be
    accessed concurrently with only mmap_sem held in read mode as noticed by
    KCSAN,

    BUG: KCSAN: data-race in swap_cluster_readahead / swap_cluster_readahead

    write to 0xffffffff92763830 of 8 bytes by task 14795 on cpu 17:
    swap_cluster_readahead+0x2a6/0x5e0
    swapin_readahead+0x92/0x8dc
    do_swap_page+0x49b/0xf20
    __handle_mm_fault+0xcfb/0xd70
    handle_mm_fault+0xfc/0x2f0
    do_page_fault+0x263/0x715
    page_fault+0x34/0x40

    1 lock held by (dnf)/14795:
    #0: ffff897bd2e98858 (&mm->mmap_sem#2){++++}-{3:3}, at: do_page_fault+0x143/0x715
    do_user_addr_fault at arch/x86/mm/fault.c:1405
    (inlined by) do_page_fault at arch/x86/mm/fault.c:1535
    irq event stamp: 83493
    count_memcg_event_mm+0x1a6/0x270
    count_memcg_event_mm+0x119/0x270
    __do_softirq+0x365/0x589
    irq_exit+0xa2/0xc0

    read to 0xffffffff92763830 of 8 bytes by task 1 on cpu 22:
    swap_cluster_readahead+0xfd/0x5e0
    swapin_readahead+0x92/0x8dc
    do_swap_page+0x49b/0xf20
    __handle_mm_fault+0xcfb/0xd70
    handle_mm_fault+0xfc/0x2f0
    do_page_fault+0x263/0x715
    page_fault+0x34/0x40

    1 lock held by systemd/1:
    #0: ffff897c38f14858 (&mm->mmap_sem#2){++++}-{3:3}, at: do_page_fault+0x143/0x715
    irq event stamp: 43530289
    count_memcg_event_mm+0x1a6/0x270
    count_memcg_event_mm+0x119/0x270
    __do_softirq+0x365/0x589
    irq_exit+0xa2/0xc0

    Signed-off-by: Qian Cai <cai@lca.pw>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Cc: Marco Elver <elver@google.com>
    Cc: Hugh Dickins <hughd@google.com>
    Link: http://lkml.kernel.org/r/20200402213748.2237-1-cai@lca.pw
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/swap_state.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/mm/swap_state.c b/mm/swap_state.c
    index 755be95d52f9c..3ceea86818bd4 100644
    --- a/mm/swap_state.c
    +++ b/mm/swap_state.c
    @@ -524,10 +524,11 @@ static unsigned long swapin_nr_pages(unsigned long offset)
    return 1;

    hits = atomic_xchg(&swapin_readahead_hits, 0);
    - pages = __swapin_nr_pages(prev_offset, offset, hits, max_pages,
    + pages = __swapin_nr_pages(READ_ONCE(prev_offset), offset, hits,
    + max_pages,
    atomic_read(&last_readahead_pages));
    if (!hits)
    - prev_offset = offset;
    + WRITE_ONCE(prev_offset, offset);
    atomic_set(&last_readahead_pages, pages);

    return pages;
    --
    2.25.1
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