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

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Message-Id: <20251220141601.e6694f904167bf12fa319a89@linux-foundation.org>
Date: Sat, 20 Dec 2025 14:16:01 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Vlastimil Babka <vbabka@...e.cz>
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 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?