Re: [PATCH 2/2] mm/page_alloc: only skip kswapd nodes below their kswapd watermark

[sashiko-bot@kernel.org]

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issue to consider:
- [Low] mm/page_alloc: missing Fixes trailer for commit 19c5fb83f2a4
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commit e932dd681884969a67ea7579103c6a9f74477b89
Author: Joshua Hahn <joshua.hahnjy@gmail.com>

> mm/page_alloc: only skip kswapd nodes below their kswapd watermark
>
> Commit 19c5fb83f2a4 ("mm: page_alloc: avoid kswapd thrashing due to
> NUMA restrictions") makes get_page_from_freelist() skip any node whose
> kswapd is active to avoid piling on an already-pressured node that is
> constantly churning to make free space that is immediately consumed.
>
> But kswapd can also be woken up for order 9 requests with THP defrag
> set to defer. This means that a fragmented node with plenty of free
> memory will have a kswapd thread continuously spinning and preventing
> order 0 pages from allocating on the node. This causes the opposite
> problem of piling on pressure on all other nodes.
>
> This behavior was observed on some Meta production workloads running on
> 2-NUMA systems, where processes with a preference for node0 allocations
> continuously allocated order 0 pages on node1, despite node0 having
> hundreds of GB of free memory.
>
> To prevent piling pressure on any one node, only skip a node during
> page allocation if the zone also fails the watermark kswapd is
> reclaiming it up to. This is the high watermark, or the promo watermark
> if NUMA balancing memory tiering is enabled. Make sure to mask the
> reserve flags so that __GFP_HIGH / atomic requests in the slowpath
> do not get a relaxed version of the check.
>
> This preserves the existing fix to prevent piling memory on the local
> node, while also preventing memory pileups on non-local nodes when
> the local node has memory.
>
> On a 2-NUMA VM with a 60G node0 and a 20G node1, with THP enabled=always
> and defrag=defer, I ran a process that prefers allocating on node0.
> With a fragmented node0:
> - Before the patch, 66% of faults were served from node0
> - After the patch, 100% of faults were served from node0
>
> Suggested-by: Daniel Hodges <hodgesd@meta.com>
> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>

[Severity: Low]
Since this fixes the unconditional node-skipping behavior introduced in
get_page_from_freelist(), should this include:

  Fixes: 19c5fb83f2a4 ("mm: page_alloc: avoid kswapd thrashing due to NUMA restrictions")

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Sashiko AI review · https://sashiko.dev/#/patchset/20261106172620.318113-1-joshua.hahnjy@gmail.com?part=2