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[2003:d8:2f25:700:d846:15f3:6ca0:8029]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e74893acecsm4580959f8f.36.2025.09.08.07.08.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 07:08:05 -0700 (PDT) Message-ID: <1f714ba0-cdda-4122-b6a1-e1e0ea44b1f2@redhat.com> Date: Mon, 8 Sep 2025 16:08:03 +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 2/7] mm/gup: check ref_count instead of lru before migration 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: <47c51c9a-140f-1ea1-b692-c4bae5d1fa58@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 12:40, Hugh Dickins wrote: > On Mon, 1 Sep 2025, David Hildenbrand wrote: >> On 31.08.25 11:05, Hugh Dickins wrote: >>> diff --git a/mm/gup.c b/mm/gup.c >>> index adffe663594d..82aec6443c0a 100644 >>> --- a/mm/gup.c >>> +++ b/mm/gup.c >>> @@ -2307,7 +2307,8 @@ static unsigned long >>> collect_longterm_unpinnable_folios( >>> continue; >>> } >>> - if (!folio_test_lru(folio) && drain_allow) { >>> + if (drain_allow && folio_ref_count(folio) != >>> + folio_expected_ref_count(folio) + 1) { >>> lru_add_drain_all(); >>> drain_allow = false; >>> } >> >> In general, to the fix idea >> >> Acked-by: David Hildenbrand > > Thanks, but I'd better not assume that in v2, even though code the same. > Will depend on how you feel about added paragraph in v2 commit message. > >> >> But as raised in reply to patch #1, we have to be a bit careful about >> including private_2 in folio_expected_ref_count() at this point. >> >> If we cannot include it in folio_expected_ref_count(), it's all going to be a >> mess until PG_private_2 is removed for good. >> >> So that part still needs to be figured out. > > Here's that added paragraph: > > Note on PG_private_2: ceph and nfs are still using the deprecated > PG_private_2 flag, with the aid of netfs and filemap support functions. > Although it is consistently matched by an increment of folio ref_count, > folio_expected_ref_count() intentionally does not recognize it, and ceph > folio migration currently depends on that for PG_private_2 folios to be > rejected. New references to the deprecated flag are discouraged, so do > not add it into the collect_longterm_unpinnable_folios() calculation: > but longterm pinning of transiently PG_private_2 ceph and nfs folios > (an uncommon case) may invoke a redundant lru_add_drain_all(). Would we also loop forever trying to migrate these folios if they reside on ZONE_MOVABLE? I would assume that is already the case, that migration will always fail due to the raised reference. -- Cheers David / dhildenb