linux-kernel - Re: [PATCH 6/7] mm: folio_may_be_cached() unless folio_test

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Message-ID: <7fe2380f-a83e-4a9e-8c5e-8459c9af0d5f@redhat.com>
Date: Mon, 1 Sep 2025 10:13:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Will Deacon <will@...nel.org>, Shivank Garg <shivankg@....com>,
 Matthew Wilcox <willy@...radead.org>, Christoph Hellwig <hch@...radead.org>,
 Keir Fraser <keirf@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>,
 John Hubbard <jhubbard@...dia.com>, Frederick Mayle <fmayle@...gle.com>,
 Peter Xu <peterx@...hat.com>, "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>,
 Alexander Krabler <Alexander.Krabler@...a.com>, Ge Yang
 <yangge1116@....com>, Li Zhe <lizhe.67@...edance.com>,
 Chris Li <chrisl@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
 Axel Rasmussen <axelrasmussen@...gle.com>, Yuanchu Xie <yuanchu@...gle.com>,
 Wei Xu <weixugc@...gle.com>, Konstantin Khlebnikov <koct9i@...il.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 6/7] mm: folio_may_be_cached() unless folio_test_large()

On 31.08.25 11:16, Hugh Dickins wrote:
> mm/swap.c and mm/mlock.c agree to drain any per-CPU batch as soon as
> a large folio is added: so collect_longterm_unpinnable_folios() just
> wastes effort when calling lru_add_drain_all() on a large folio.
> 
> But although there is good reason not to batch up PMD-sized folios,
> we might well benefit from batching a small number of low-order mTHPs
> (though unclear how that "small number" limitation will be implemented).
> 
> So ask if folio_may_be_cached() rather than !folio_test_large(), to
> insulate those particular checks from future change.  Name preferred
> to "folio_is_batchable" because large folios can well be put on a batch:
> it's just the per-CPU LRU caches, drained much later, which need care.
> 
> Marked for stable, to counter the increase in lru_add_drain_all()s
> from "mm/gup: check ref_count instead of lru before migration".
> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: <stable@...r.kernel.org>
> ---
>   include/linux/swap.h | 10 ++++++++++
>   mm/gup.c             |  5 +++--
>   mm/mlock.c           |  6 +++---
>   mm/swap.c            |  2 +-
>   4 files changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 2fe6ed2cc3fd..b49a61c32238 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -385,6 +385,16 @@ void folio_add_lru_vma(struct folio *, struct vm_area_struct *);
>   void mark_page_accessed(struct page *);
>   void folio_mark_accessed(struct folio *);
>   

Two smaller things:

(1) We have other "folio_maybe_*" functions, so this one should likely
     better start with that as well.

(2) With things like fscache in mind, the function can be a bit
     misleading.

So I wonder if (a) we should just add kerneldoc to document it clearly 
(lru cache, mlock cache?) and (b) maybe call it 
folio_may_be_lru_cached(). Not sure if we can find a better abstraction 
for these two caches.

Thinking again, "maybe_cached" might be a bit misleading because it 
implements a very very very bad heuristic for small folios.

Maybe it's more like "supports being cached".

folio_lru_caching_supported()

Something like that, maybe? (again, unclear about lru/mlock cache 
abstraction)


-- 
Cheers

David / dhildenb