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    SubjectRe: [PATCH] mm: avoid livelock on !__GFP_FS allocations
    On Tue, Oct 25, 2011 at 02:26:56AM -0700, Colin Cross wrote:
    > On Tue, Oct 25, 2011 at 2:09 AM, Mel Gorman <mgorman@suse.de> wrote:
    > > On Mon, Oct 24, 2011 at 11:39:49PM -0700, Colin Cross wrote:
    > >> Under the following conditions, __alloc_pages_slowpath can loop
    > >> forever:
    > >> gfp_mask & __GFP_WAIT is true
    > >> gfp_mask & __GFP_FS is false
    > >> reclaim and compaction make no progress
    > >> order <= PAGE_ALLOC_COSTLY_ORDER
    > >>
    > >> These conditions happen very often during suspend and resume,
    > >> when pm_restrict_gfp_mask() effectively converts all GFP_KERNEL
    > >> allocations into __GFP_WAIT.
    > > b>
    > >> The oom killer is not run because gfp_mask & __GFP_FS is false,
    > >> but should_alloc_retry will always return true when order is less
    > >> than PAGE_ALLOC_COSTLY_ORDER.
    > >>
    > >> Fix __alloc_pages_slowpath to skip retrying when oom killer is
    > >> not allowed by the GFP flags, the same way it would skip if the
    > >> oom killer was allowed but disabled.
    > >>
    > >> Signed-off-by: Colin Cross <ccross@android.com>
    > >
    > > Hi Colin,
    > >
    > > Your patch functionally seems fine. I see the problem and we certainly
    > > do not want to have the OOM killer firing during suspend. I would prefer
    > > that the IO devices would not be suspended until reclaim was completed
    > > but I imagine that would be a lot harder.
    > >
    > > That said, it will be difficult to remember why checking __GFP_NOFAIL in
    > > this case is necessary and someone might "optimitise" it away later. It
    > > would be preferable if it was self-documenting. Maybe something like
    > > this? (This is totally untested)
    >
    > This issue is not limited to suspend, any GFP_NOIO allocation could
    > end up in the same loop. Suspend is the most likely case, because it
    > effectively converts all GFP_KERNEL allocations into GFP_NOIO.
    >

    I see what you mean with GFP_NOIO but there is an important difference
    between GFP_NOIO and suspend. A GFP_NOIO low-order allocation currently
    implies __GFP_NOFAIL as commented on in should_alloc_retry(). If no progress
    is made, we call wait_iff_congested() and sleep for a bit. As the system
    is running, kswapd and other process activity will proceed and eventually
    reclaim enough pages for the GFP_NOIO allocation to succeed. In a running
    system, GFP_NOIO can stall for a period of time but your patch will cause
    the allocation to fail. While I expect callers return ENOMEM or handle
    the situation properly with a wait-and-retry loop, there will be
    operations that fail that used to succeed. This is why I'd prefer it was
    a suspend-specific fix unless we know there is a case where a machine
    livelocks due to a GFP_NOIO allocation looping forever and even then I'd
    wonder why kswapd was not helping.

    --
    Mel Gorman
    SUSE Labs


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