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b=cBRX+Koh7WcZBHXOpfa3OnkSGgM2W32Agea71Iv7jFYkqGDqyceCKtM7zfZiUwseu /CQ9jXfiYpIXNuLlT58tEWkIYDmFUsGw7CkROCqPz3otdwnqUxO6X/5HqvQD6g00cK QDyCOKrUuKod61GeP02OiBW6WeaBrxAEtPaamM1ob/0HkrRHYiXjjN61WPnDL3/qEf O8dR3E4RZXskm5fVpG3vAs29z6Y9LDJnzbOYiVKQyk7J7VT0gsjV2fqzTbXVucTvE7 BNHViop4QCQ35Sqpvxe8lmD9hedEzKes+dogSa/9EMeyrlp0aHH4+KZKc3We2Q/B+v NPNtKcr/oogGw== Date: Wed, 2 Apr 2025 18:02:04 -0700 From: Luis Chamberlain To: Jan Kara Cc: brauner@kernel.org, tytso@mit.edu, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, riel@surriel.com, willy@infradead.org, hannes@cmpxchg.org, oliver.sang@intel.com, dave@stgolabs.net, david@redhat.com, axboe@kernel.dk, hare@suse.de, david@fromorbit.com, djwong@kernel.org, ritesh.list@gmail.com, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-mm@kvack.org, gost.dev@samsung.com, p.raghav@samsung.com, da.gomez@samsung.com Subject: Re: [PATCH 2/3] fs/buffer: avoid races with folio migrations on __find_get_block_slow() Message-ID: References: <20250330064732.3781046-1-mcgrof@kernel.org> <20250330064732.3781046-3-mcgrof@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 5B9F5100008 X-Stat-Signature: 8xa9pomyarftnjs6syoz1f5kitgj73mo X-HE-Tag: 1743642129-254982 X-HE-Meta: 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 +iySSTwW 9fIulLzGkHPB9k9pLdr/hcTFcIhgAGXY7cBzOBhZa8heHpVuS9m3VIS0QRse8WpX+gnisnkNN5i6ARZ0RQqpI2qRbqEicGJKpZsFD0HR+7oIBOM05I6gMyaDSNGhr4BIqL8nayKDzumMzl40aWojDU1+4HbVYvGChwONdmwnPgio6sx1KAWRS+ofezuTOl/vO2/9zFiFcHOOeZay4Lj7ACBz0Jhy4VbSiE2s8uLeVwLpQ41vsGq7glciVugE8ExKD3TT8pNlfMiqlNGqmpRD2yeZCnHuTfZZin9TyQxtsQ2J0av9OGfXZ+Itw5yyPPxbK6KQ/w3a/rkrFrUA= 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 Tue, Apr 01, 2025 at 12:57:37PM +0200, Jan Kara wrote: > On Sat 29-03-25 23:47:31, Luis Chamberlain wrote: > > diff --git a/fs/buffer.c b/fs/buffer.c > > index c7abb4a029dc..a4e4455a6ce2 100644 > > --- a/fs/buffer.c > > +++ b/fs/buffer.c > > @@ -208,6 +208,15 @@ __find_get_block_slow(struct block_device *bdev, sector_t block) > > head = folio_buffers(folio); > > if (!head) > > goto out_unlock; > > + > > + if (folio->mapping->a_ops->migrate_folio && > > + folio->mapping->a_ops->migrate_folio == buffer_migrate_folio_norefs) { > > This is always true for bdev mapping we have here, isn't it? Yes, thanks! > > + if (folio_test_lru(folio) && > > Do you expect bdev page cache to contain non-LRU folios? I thought every > pagecache folio is on LRU so this seems pointless as well? But I may be > missing something here. > > > + folio_test_locked(folio) && > > + !folio_test_writeback(folio)) > > + goto out_unlock; > > I find this problematic. It fixes the race with migration, alright > (although IMO we should have a comment very well explaining the interplay > of folio lock and mapping->private_lock to make this work - probably in > buffer_migrate_folio_norefs() - and reference it from here), but there are > places which expect that if __find_get_block() doesn't return anything, > this block is not cached in the buffer cache. And your change breaks this > assumption. Yup agreed! Luis