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b=mCrlU9qnLmCDoMjoYWy2y0kteLWFvhGmH4vE46dQiE+IlMGWWq8McDNVEzD9elWx3 juv4qEb/MC/XNLRAklmjfLHtMHpTTvfP7oFgME+Rhl27NzU6spRtGwNmfWK4EEyMqT 9t3bYLGlSPN9dEP7JKVf14K131nuy0Q27ZhwLWf09RqEwaiVKjfypqlxLrzG+VZn9V rO67iYJpwiEMYMyxDQi1csCNK9oWDQzBQMnxI41NR1WVZGTFszMidaaQcA26nTZMBo 6XmAJt7zrmAJTzv9Rogz+Ve8y2GeiqFxsBr2iucOrW+9M6FNqTwvnWC3DRDfNM48FF BQ9vTfkzJLoqg== Message-ID: Date: Fri, 21 Aug 2026 16:18:31 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: "David Hildenbrand (Arm)" Subject: Re: [RFC PATCH v3 0/4] mm: enable lru cache for smaller large folios To: Barry Song Cc: akpm@linux-foundation.org, linux-mm@kvack.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, ljs@kernel.org, mhocko@suse.com, npache@redhat.com, rppt@kernel.org, ryan.roberts@arm.com, surenb@google.com, vbabka@kernel.org, ziy@nvidia.com, hughd@google.com, ackerleytng@google.com, usama.arif@linux.dev, joannelkoong@gmail.com, hannes@cmpxchg.org References: <20260818225904.55236-1-baohua@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: wk9zampa6w58jbhdoro9rpbowqgwjshe X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: DE0D920009 X-HE-Tag: 1787321918-301830 X-HE-Meta: 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 oNIoT5zQ nw5SzPnHEl5eSxbnO51c4IOludj0NTO3prV7fV20Qf5wivASh4MjmKQ2nTZzeJH36GxpRpQJ7TxTjZ+ADMm1ELQtmadCZoQleT9pj/ZXGMufUE1Z3itikFk9i1eujpewedHEpJRD+HcQwwXzwue0egoOLeogLE6lO1h+1jioj58W7kGqmCDnTJcud5O6pQgOKAvhq2jnfhNtb/zMHGoPEbb/pL+DVbiGQoppmKjXfjyLdMGgOCHrG/nj2xEKJ1Dh8oRb22Ovkcnk+5m0j560KB6vI7xdUeW8im65FRbuzwUPEpD1gwgwL7rOAPA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/20/26 22:18, Barry Song wrote: > On Fri, Aug 21, 2026 at 2:23 AM David Hildenbrand (Arm) > wrote: >> >> On 8/19/26 00:59, Barry Song (Xiaomi) wrote: >>> This patchset enables the per-CPU LRU cache for large folios with fewer >>> than `FOLIO_BATCH_SIZE` (31) pages. It also limits each per-CPU LRU cache >>> to at most `FOLIO_BATCH_SIZE` pages to avoid negatively affecting >>> accounting and memory reclamation pressure. >>> >>> This is particularly beneficial on systems that use relatively small >>> large folios. For larger folios, the benefit is likely to be smaller >>> because far fewer folios are expected to contend for the LRU cache. >> >> As raised, there is this problem with collect_longterm_unpinnable_folios() >> >> (see >> https://lore.kernel.org/r/20260806-lru_cache_drain_for_folio-v1-1-c6287d295e99@kernel.org >> ) >> >> whereby we don't know how many refs we actually hold. Certainly not 1. >> >> We might have to wait for Hugh's cleanup to handle that cleanly (and avoid all >> the other LRU cache draining). > > Hi David, > Thanks for raising this. > Yes, I saw your comment and took a closer look at it. I think the > best approach for now is to leave that part untouched until Hugh's > patch lands? I think we should not consider your patch set until Hugh either resolved it or we have a better way to handle draining. Building on top of this for large folios now just gets painful. > We might end up doing some extra draining in that case, > but that's safe—any value greater than 1 might not be. We'll unconditionally drain all LRU caches, which is precisely *not* what we want to do in the first place. :) -- Cheers, David