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[2003:d8:2f23:9c00:d1f6:f7fe:8f14:7e34]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45dd2304e16sm224325475e9.7.2025.09.09.00.54.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Sep 2025 00:54:07 -0700 (PDT) Message-ID: <1bbaa34f-6c32-46a5-bcc9-f8886b865d18@redhat.com> Date: Tue, 9 Sep 2025 09:54:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/6] mm/gup: check ref_count instead of lru before migration To: Hugh Dickins , Andrew Morton Cc: Alexander Krabler , "Aneesh Kumar K.V" , Axel Rasmussen , Chris Li , Christoph Hellwig , Frederick Mayle , Jason Gunthorpe , Johannes Weiner , John Hubbard , Keir Fraser , Konstantin Khlebnikov , Li Zhe , Matthew Wilcox , Peter Xu , Rik van Riel , Shivank Garg , Vlastimil Babka , Wei Xu , Will Deacon , yangge , Yuanchu Xie , Yu Zhao , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <41395944-b0e3-c3ac-d648-8ddd70451d28@google.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 1FE3E14000C X-Stat-Signature: fwnmhs55bogwiboimgzccubik8cw85ym X-Rspam-User: X-HE-Tag: 1757404450-318546 X-HE-Meta: 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 /P1gHBKY UhiBcXG+ifSEkzAlErxLyR1vuvAB8jvxHFOxv/iLez9U+GEEGerCGhHsnGeHSyvSCIk9ZgNv5qJh7gSvPUT3VpNC/YsmAphvLJzgEHdO6z28WEHRafcD0ZoYJCC1FOW1h4lgVsfs2snXkZqB5+ByLkVPOHAWLR/xUEt9tKR0XqW18pkjoxm2LFWd8Ju8iDYEk5z4l+we5now2QfwzxON7IMv5Wg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09.09.25 00:15, Hugh Dickins wrote: > Will Deacon reports:- > > When taking a longterm GUP pin via pin_user_pages(), > __gup_longterm_locked() tries to migrate target folios that should not > be longterm pinned, for example because they reside in a CMA region or > movable zone. This is done by first pinning all of the target folios > anyway, collecting all of the longterm-unpinnable target folios into a > list, dropping the pins that were just taken and finally handing the > list off to migrate_pages() for the actual migration. > > It is critically important that no unexpected references are held on the > folios being migrated, otherwise the migration will fail and > pin_user_pages() will return -ENOMEM to its caller. Unfortunately, it is > relatively easy to observe migration failures when running pKVM (which > uses pin_user_pages() on crosvm's virtual address space to resolve > stage-2 page faults from the guest) on a 6.15-based Pixel 6 device and > this results in the VM terminating prematurely. > > In the failure case, 'crosvm' has called mlock(MLOCK_ONFAULT) on its > mapping of guest memory prior to the pinning. Subsequently, when > pin_user_pages() walks the page-table, the relevant 'pte' is not > present and so the faulting logic allocates a new folio, mlocks it > with mlock_folio() and maps it in the page-table. > > Since commit 2fbb0c10d1e8 ("mm/munlock: mlock_page() munlock_page() > batch by pagevec"), mlock/munlock operations on a folio (formerly page), > are deferred. For example, mlock_folio() takes an additional reference > on the target folio before placing it into a per-cpu 'folio_batch' for > later processing by mlock_folio_batch(), which drops the refcount once > the operation is complete. Processing of the batches is coupled with > the LRU batch logic and can be forcefully drained with > lru_add_drain_all() but as long as a folio remains unprocessed on the > batch, its refcount will be elevated. > > This deferred batching therefore interacts poorly with the pKVM pinning > scenario as we can find ourselves in a situation where the migration > code fails to migrate a folio due to the elevated refcount from the > pending mlock operation. > > Hugh Dickins adds:- > > !folio_test_lru() has never been a very reliable way to tell if an > lru_add_drain_all() is worth calling, to remove LRU cache references > to make the folio migratable: the LRU flag may be set even while the > folio is held with an extra reference in a per-CPU LRU cache. > > 5.18 commit 2fbb0c10d1e8 may have made it more unreliable. Then 6.11 > commit 33dfe9204f29 ("mm/gup: clear the LRU flag of a page before adding > to LRU batch") tried to make it reliable, by moving LRU flag clearing; > but missed the mlock/munlock batches, so still unreliable as reported. > > And it turns out to be difficult to extend 33dfe9204f29's LRU flag > clearing to the mlock/munlock batches: if they do benefit from batching, > mlock/munlock cannot be so effective when easily suppressed while !LRU. > > Instead, switch to an expected ref_count check, which was more reliable > all along: some more false positives (unhelpful drains) than before, and > never a guarantee that the folio will prove migratable, but better. > > 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(). And > this makes easy the backport to earlier releases: up to and including > 6.12, btrfs also used PG_private_2, but without a ref_count increment. > > Note for stable backports: requires 6.16 commit 86ebd50224c0 ("mm: > add folio_expected_ref_count() for reference count calculation"). > > Reported-by: Will Deacon > Closes: https://lore.kernel.org/linux-mm/20250815101858.24352-1-will@kernel.org/ > Fixes: 9a4e9f3b2d73 ("mm: update get_user_pages_longterm to migrate pages allocated from CMA region") > Signed-off-by: Hugh Dickins > Cc: > --- > mm/gup.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > 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; > } Acked-by: David Hildenbrand -- Cheers David / dhildenb