• Eric W. Biederman's avatar
    fs/file.c:fdtable: avoid triggering OOMs from alloc_fdmem · 434672c0
    Eric W. Biederman authored
    commit 96c7a2ff21501691587e1ae969b83cbec8b78e08 upstream.
    
    Recently due to a spike in connections per second memcached on 3
    separate boxes triggered the OOM killer from accept.  At the time the
    OOM killer was triggered there was 4GB out of 36GB free in zone 1.  The
    problem was that alloc_fdtable was allocating an order 3 page (32KiB) to
    hold a bitmap, and there was sufficient fragmentation that the largest
    page available was 8KiB.
    
    I find the logic that PAGE_ALLOC_COSTLY_ORDER can't fail pretty dubious
    but I do agree that order 3 allocations are very likely to succeed.
    
    There are always pathologies where order > 0 allocations can fail when
    there are copious amounts of free memory available.  Using the pigeon
    hole principle it is easy to show that it requires 1 page more than 50%
    of the pages being free to guarantee an order 1 (8KiB) allocation will
    succeed, 1 page more than 75% of the pages being free to guarantee an
    order 2 (16KiB) allocation will succeed and 1 page more than 87.5% of
    the pages being free to guarantee an order 3 allocate will succeed.
    
    A server churning memory with a lot of small requests and replies like
    memcached is a common case that if anything can will skew the odds
    against large pages being available.
    
    Therefore let's not give external applications a practical way to kill
    linux server applications, and specify __GFP_NORETRY to the kmalloc in
    alloc_fdmem.  Unless I am misreading the code and by the time the code
    reaches should_alloc_retry in __alloc_pages_slowpath (where
    __GFP_NORETRY becomes signification).  We have already tried everything
    reasonable to allocate a page and the only thing left to do is wait.  So
    not waiting and falling back to vmalloc immediately seems like the
    reasonable thing to do even if there wasn't a chance of triggering the
    OOM killer.
    Signed-off-by: 's avatar"Eric W. Biederman" <ebiederm@xmission.com>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Acked-by: 's avatarDavid Rientjes <rientjes@google.com>
    Cc: Cong Wang <cwang@twopensource.com>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    434672c0
file.c 21.9 KB