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[2003:d8:2f25:700:d846:15f3:6ca0:8029]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e18537589dsm16164452f8f.54.2025.09.08.07.35.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 07:35:18 -0700 (PDT) Message-ID: <3f76748b-8f99-4e63-ba39-adadc2f58838@redhat.com> Date: Mon, 8 Sep 2025 16:35:16 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 6/7] mm: folio_may_be_cached() unless folio_test_large() To: Hugh Dickins Cc: Andrew Morton , Will Deacon , Shivank Garg , Matthew Wilcox , Christoph Hellwig , Keir Fraser , Jason Gunthorpe , John Hubbard , Frederick Mayle , Peter Xu , "Aneesh Kumar K.V" , Johannes Weiner , Vlastimil Babka , Alexander Krabler , Ge Yang , Li Zhe , Chris Li , Yu Zhao , Axel Rasmussen , Yuanchu Xie , Wei Xu , Konstantin Khlebnikov , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <861c061c-51cd-b940-49df-9f55e1fee2c8@google.com> <7fe2380f-a83e-4a9e-8c5e-8459c9af0d5f@redhat.com> <7113d289-fb8e-4589-7eb5-1f7139965ade@google.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 08.09.25 13:19, Hugh Dickins wrote: > On Mon, 1 Sep 2025, David Hildenbrand wrote: >> 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 >>> Signed-off-by: Hugh Dickins >>> Cc: >>> --- >>> 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() > > folio_may_be_cached() -> folio_may_be_lru_cached(), yes, that's > very much better, thanks. > > (Settimg aside that I've never perceived those pagevecs/batches as a > "cache"; but lru_cache_disable() gave us that terminology, and we've > gone with the flow ever since. lru_add_drain() would be better named > lru_cache_drain() now, I've always got hung up on "adding a drain".) Yeah, the terminology is not that intuitive :) Not sure if using "batched" instead of "cached" might be clearer long-term? > > "may be" rather than "maybe" was intentional: perhaps too subtle, > but to a native speaker it neatly expresses both the "we can do this" > and "might this have been done" cases. I would wish we could find something that also non-native speakers can immediately understand ;) "may_get_lru_cached" / "may_get_lru_batched"? /me could not even phrase it in German properly > > kernel-doc? I don't think so, this is very much an mm-internal > matter, and I don't care for the way kernel-doc forces us towards > boilerplate ("@folio: The folio.") rather than helpful comment. So a comment that this is an internal helper might be nice. Or we just move it straight to mm/internal.h ? -- Cheers David / dhildenb