• Vlastimil Babka's avatar
    fs/select: add vmalloc fallback for select(2) · 2d19309c
    Vlastimil Babka authored
    The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
    with the number of fds passed. We had a customer report page allocation
    failures of order-4 for this allocation. This is a costly order, so it might
    easily fail, as the VM expects such allocation to have a lower-order fallback.
    
    Such trivial fallback is vmalloc(), as the memory doesn't have to be physically
    contiguous and the allocation is temporary for the duration of the syscall
    only. There were some concerns, whether this would have negative impact on the
    system by exposing vmalloc() to userspace. Although an excessive use of vmalloc
    can cause some system wide performance issues - TLB flushes etc. - a large
    order allocation is not for free either and an excessive reclaim/compaction can
    have a similar effect. Also note that the size is effectively limited by
    RLIMIT_NOFILE which defaults to 1024 on the systems I checked. That means the
    bitmaps will fit well within single page and thus the vmalloc() fallback could
    be only excercised for processes where root allows a higher limit.
    
    Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
    it doesn't need this kind of fallback.
    
    [eric.dumazet@gmail.com: fix failure path logic]
    [akpm@linux-foundation.org: use proper type for size]
    Link: http://lkml.kernel.org/r/20160927084536.5923-1-vbabka@suse.czSigned-off-by: 's avatarVlastimil Babka <vbabka@suse.cz>
    Acked-by: 's avatarMichal Hocko <mhocko@suse.com>
    Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: David Laight <David.Laight@ACULAB.COM>
    Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
    Cc: Nicholas Piggin <npiggin@gmail.com>
    Cc: Jason Baron <jbaron@akamai.com>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    2d19309c