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Subject: Re: [PATCH 1/2] mm: thp: relax __GFP_THISNODE for MADV_HUGEPAGE
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To:     Andrea Arcangeli <aarcange@redhat.com>,
        David Rientjes <rientjes@google.com>
Cc:     Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Argangeli <andrea@kernel.org>,
        Zi Yan <zi.yan@cs.rutgers.edu>,
        Stefan Priebe - Profihost AG <s.priebe@profihost.ag>,
        "Kirill A. Shutemov" <kirill@shutemov.name>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Stable tree <stable@vger.kernel.org>,
        Michal Hocko <mhocko@suse.com>
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Message-ID: <53169158-09b1-e592-def0-de5e4c47ce16@suse.cz>
Date:   Wed, 10 Oct 2018 09:54:32 +0200
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On 10/10/18 12:51 AM, Andrea Arcangeli wrote:
> Yes there's one case where reclaim is "pointless", but it happens once
> and then COMPACT_DEFERRED is returned and __GFP_NORETRY will skip
> reclaim then.
> 
> So you're right when we hit fragmentation there's one and only one
> "pointless" reclaim invocation. And immediately after we also
> exponentially backoff on the compaction invocations with the
> compaction deferred logic.
> 
> We could try optimize away such "pointless" reclaim event for sure,
> but it's probably an optimization that may just get lost in the noise
> and may not be measurable, because it only happens once when the first
> full fragmentation is encountered.

Note there's a small catch in the above. defer_compaction() has always
only been called after a failure on higher priority than
COMPACT_PRIO_ASYNC, where it's assumed that async compaction can
terminate prematurely due to a number of reasons, so it doesn't mean
that the zone itself cannot be compacted.
And, for __GFP_NORETRY, if the initial compaction fails, we keep using
async compaction also for the second, after-reclaim attempt (which would
otherwise use SYNC_LIGHT):

        /*
         * Looks like reclaim/compaction is worth trying, but
         * sync compaction could be very expensive, so keep
         * using async compaction.
         */
        compact_priority = INIT_COMPACT_PRIORITY;

This doesn't affect current madvised THP allocation which doesn't use
__GFP_NORETRY, but could explain why you saw no benefit from changing it
to __GFP_NORETRY.