linux-kernel - Re: [PATCH RFC 1/2] mm, page_alloc, thp: prevent reclaim for __GFP

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Message-ID: <xow3biulatuampfmzfbmnmrc4lkzgbhys76gpwrxrdwesr3y6q@c6bpczjcgact>
Date: Wed, 17 Dec 2025 15:53:52 +0000
From: Pedro Falcato <pfalcato@...e.de>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	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>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 1/2] mm, page_alloc, thp: prevent reclaim for
 __GFP_THISNODE THP allocations

On Tue, Dec 16, 2025 at 04:54:21PM +0100, Vlastimil Babka 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.
> 
> Fixes: 2f0799a0ffc0 ("mm, thp: restore node-local hugepage allocations")
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Pedro Falcato <pfalcato@...e.de> 

-- 
Pedro