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    SubjectRe: [PATCH v3 03/17] mm, page_alloc: don't retry initial attempt in slowpath
    On Fri 24-06-16 11:54:23, Vlastimil Babka wrote:
    > After __alloc_pages_slowpath() sets up new alloc_flags and wakes up kswapd, it
    > first tries get_page_from_freelist() with the new alloc_flags, as it may
    > succeed e.g. due to using min watermark instead of low watermark. It makes
    > sense to to do this attempt before adjusting zonelist based on
    > alloc_flags/gfp_mask, as it's still relatively a fast path if we just wake up
    > kswapd and successfully allocate.
    >
    > This patch therefore moves the initial attempt above the retry label and
    > reorganizes a bit the part below the retry label. We still have to attempt
    > get_page_from_freelist() on each retry, as some allocations cannot do that
    > as part of direct reclaim or compaction, and yet are not allowed to fail
    > (even though they do a WARN_ON_ONCE() and thus should not exist). We can reuse
    > the call meant for ALLOC_NO_WATERMARKS attempt and just set alloc_flags to
    > ALLOC_NO_WATERMARKS if the context allows it. As a side-effect, the attempts
    > from direct reclaim/compaction will also no longer obey watermarks once this
    > is set, but there's little harm in that.
    >
    > Kswapd wakeups are also done on each retry to be safe from potential races
    > resulting in kswapd going to sleep while a process (that may not be able to
    > reclaim by itself) is still looping.
    >
    > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

    Acked-by: Michal Hocko <mhocko@suse.com>

    > ---
    > mm/page_alloc.c | 29 ++++++++++++++++++-----------
    > 1 file changed, 18 insertions(+), 11 deletions(-)
    >
    > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > index 82545274adbe..06cfa4bb807d 100644
    > --- a/mm/page_alloc.c
    > +++ b/mm/page_alloc.c
    > @@ -3582,35 +3582,42 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
    > */
    > alloc_flags = gfp_to_alloc_flags(gfp_mask);
    >
    > + if (gfp_mask & __GFP_KSWAPD_RECLAIM)
    > + wake_all_kswapds(order, ac);
    > +
    > + /*
    > + * The adjusted alloc_flags might result in immediate success, so try
    > + * that first
    > + */
    > + page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
    > + if (page)
    > + goto got_pg;
    > +
    > +
    > retry:
    > + /* Ensure kswapd doesn't accidentally go to sleep as long as we loop */
    > if (gfp_mask & __GFP_KSWAPD_RECLAIM)
    > wake_all_kswapds(order, ac);
    >
    > + if (gfp_pfmemalloc_allowed(gfp_mask))
    > + alloc_flags = ALLOC_NO_WATERMARKS;
    > +
    > /*
    > * Reset the zonelist iterators if memory policies can be ignored.
    > * These allocations are high priority and system rather than user
    > * orientated.
    > */
    > - if (!(alloc_flags & ALLOC_CPUSET) || gfp_pfmemalloc_allowed(gfp_mask)) {
    > + if (!(alloc_flags & ALLOC_CPUSET) || (alloc_flags & ALLOC_NO_WATERMARKS)) {
    > ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
    > ac->preferred_zoneref = first_zones_zonelist(ac->zonelist,
    > ac->high_zoneidx, ac->nodemask);
    > }
    >
    > - /* This is the last chance, in general, before the goto nopage. */
    > + /* Attempt with potentially adjusted zonelist and alloc_flags */
    > page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
    > if (page)
    > goto got_pg;
    >
    > - /* Allocate without watermarks if the context allows */
    > - if (gfp_pfmemalloc_allowed(gfp_mask)) {
    > -
    > - page = get_page_from_freelist(gfp_mask, order,
    > - ALLOC_NO_WATERMARKS, ac);
    > - if (page)
    > - goto got_pg;
    > - }
    > -
    > /* Caller is not willing to reclaim, we can't balance anything */
    > if (!can_direct_reclaim) {
    > /*
    > --
    > 2.8.4
    >
    > --
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    --
    Michal Hocko
    SUSE Labs

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