linux-kernel - Re: [PATCH] mm, page_alloc, thp: prevent reclaim for __GFP

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Message-ID: <9fcd1188-8ad7-4c7b-afd8-647d97b7aae1@suse.cz>
Date: Sun, 21 Dec 2025 21:43:02 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
 Zi Yan <ziy@...dia.com>, David Rientjes <rientjes@...gle.com>,
 David Hildenbrand <david@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Mike Rapoport
 <rppt@...nel.org>, Joshua Hahn <joshua.hahnjy@...il.com>,
 Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, page_alloc, thp: prevent reclaim for __GFP_THISNODE
 THP allocations

On 12/20/25 23:16, Andrew Morton wrote:
> On Fri, 19 Dec 2025 17:31:57 +0100 Vlastimil Babka <vbabka@...e.cz> wrote:
> 
>> Since commit cc638f329ef6 ("mm, thp: tweak reclaim/compaction effort of
>> local-only and all-node allocations"), THP page fault allocations have
>> settled on the following scheme (from the commit log):
>> 
>> 1. local node only THP allocation with no reclaim, just compaction.
>> 2. for madvised VMA's or when synchronous compaction is enabled always - THP
>>    allocation from any node with effort determined by global defrag setting
>>    and VMA madvise
>> 3. fallback to base pages on any node
>> 
>> Recent customer reports however revealed we have a gap in step 1 above.
>> What we have seen is excessive reclaim due to THP page faults on a NUMA
>> node that's close to its high watermark, while other nodes have plenty
>> of free memory.
>> 
>> The problem with step 1 is that it promises no reclaim after the
>> compaction attempt, however reclaim is only avoided for certain
>> compaction outcomes (deferred, or skipped due to insufficient free base
>> pages), and not e.g. when compaction is actually performed but fails (we
>> did see compact_fail vmstat counter increasing).
>> 
>> THP page faults can therefore exhibit a zone_reclaim_mode-like behavior,
>> which is not the intention.
>> 
>> Thus add a check for __GFP_THISNODE that corresponds to this exact
>> situation and prevents continuing with reclaim/compaction once the
>> initial compaction attempt isn't successful in allocating the page.
>> 
>> Note that commit cc638f329ef6 has not introduced this over-reclaim
>> possibility; it appears to exist in some form since commit 2f0799a0ffc0
>> ("mm, thp: restore node-local hugepage allocations"). Followup commits
>> b39d0ee2632d ("mm, page_alloc: avoid expensive reclaim when compaction
>> may not succeed") and cc638f329ef6 have moved in the right direction,
>> but left the abovementioned gap.
> 
> Cool.  What are your thoughts on which kernel version(s) should receive
> this?

We could just cc stable and let Fixes: do the job. Should be safe to backport.