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[87.161.174.152]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45decf817efsm6223815e9.18.2025.09.08.13.17.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 13:17:23 -0700 (PDT) Message-ID: <919ab2ee-6493-4415-a75b-e1a2b08c0d3e@redhat.com> Date: Mon, 8 Sep 2025 22:17:21 +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> <1f714ba0-cdda-4122-b6a1-e1e0ea44b1f2@redhat.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 21:57, Hugh Dickins wrote: > On Mon, 8 Sep 2025, David Hildenbrand wrote: >> 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. > > Loop around forever? That would be unfortunate (but I presume killable). > But when I looked, it appeared that any failure of migrate_pages() there > gets reported as -ENOMEM, which would end up as an OOM? But you know > mm/gup.c very much better than I do. Yes, like I expected, we just bail out. __gup_longterm_locked() will not retry in that case. It's interesting that any migration failure is treated as -ENOMEM, but well, that's certainly material for a completely different discussion. Thanks Hugh! -- Cheers David / dhildenb