linux-kernel - Re: [PATCH v5 0/3] mm/page_alloc: Batch callers of free_pcppages

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Message-ID: <6d62386c-242a-4735-add3-fee9c6e0ba96@suse.cz>
Date: Tue, 14 Oct 2025 19:54:28 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Joshua Hahn <joshua.hahnjy@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Chris Mason <clm@...com>, Kiryl Shutsemau <kirill@...temov.name>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Brendan Jackman <jackmanb@...gle.com>, David Hildenbrand <david@...hat.com>,
 Johannes Weiner <hannes@...xchg.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Michal Hocko
 <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Zi Yan <ziy@...dia.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, kernel-team@...a.com
Subject: Re: [PATCH v5 0/3] mm/page_alloc: Batch callers of free_pcppages_bulk

On 10/14/25 16:50, Joshua Hahn wrote:
> Changelog
> =========
> v4 --> v5:
> - Wordsmithing
> - Patches 1/3 and 2/3 were left untouched.
> - Patch 3/3 no longer checks for the to_free == 0 case. It also now checks
>   for pcp->count > 0 as the condition inside the while loop, and the early
>   break checks for the opposite condition. Note that both to_free and
>   pcp->count can become negative due to high-order pages that are freed, so
>   we must check for (to_free <= 0 || pcp->count <= 0), instead of just
>   checking for == 0.

I don't see how that's possible?
- to_free is decremented by to_free_batched = min(to_free, batch); so it
can't go negative.
- pcp->count indeed decrements by nr_pages but it should be exactly zero
once pcp becomes empty. It's true that internally in free_pcppages_bulk()
the count parameter (where we pass to_free_batched) can go negative, but
that doesn't affect to_free_batched in the caller free_frozen_page_commit().
So testing for <= is unnecessary and only looks weird?