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    SubjectRe: [PATCH] mm: avoid livelock on !__GFP_FS allocations
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    On Tue, Oct 25, 2011 at 4:23 AM, Mel Gorman <mgorman@suse.de> wrote:
    > 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.

    OK, I see the change in behavior you are trying to avoid. With your
    patch GFP_NOIO allocations can still fail during suspend, is that OK?
    I'm also worried about GFP_NOIO allocations looping forever when swap
    is not enabled, but I've never seen it happen, and it would probably
    recover eventually when another tried tried a GFP_KERNEL allocation
    and oom killed something.
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