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C7DEE6021E; Wed, 21 Oct 2026 10:53:11 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7363C1F000C9; Wed, 21 Oct 2026 10:53:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792579991; bh=gMLVnnDM9TfSfMpUppQf1/87O9OPgyCoy93iqIcO2rs=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=bD0aREKlvvHtB8UL3zZ3KTvP55m3iQg7PJbTukXdUEHg7Z2dHxMz4ywzNVpgDXmA4 x0N631ipCR7GDV25tqOWNUBqXAPzTNxWGQ5PKl34pdD/kQdVkokz58K14imlZpWH3p kIJb34nBl/rO/8mBwmO0H52d/bqjQpAy36u+owlCnqRPLI4ZYwsmaWJaf/FimC2Iy5 b3+/T4+hcAU00TFo/fhb1JlccM9ww5SARPvkUN5LuLgdEpaXRcvZl5VfrxJxpIkVjd jqfLdDUxdf1z45ir3D+Pwr6MqdpoZNUO18owuqfJTkSn+uYyf3D+WfGVzQ0jWvKw64 7oPVl0BgaQdiA== Message-ID: <8b666ab283f58e5913b3e4c33c52742a1153e9bd.camel@kernel.org> Subject: Re: [PATCH v7 2/5] mm: add folio_mark_dropbehind() to mark a folio dropbehind and account it From: Jeff Layton To: Mike Snitzer , Chuck Lever Cc: linux-nfs@vger.kernel.org, NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Christoph Hellwig , Jens Axboe , Matthew Wilcox , Jonathan Corbet , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org Date: Wed, 21 Oct 2026 06:52:59 -0400 In-Reply-To: <20261020220346.28020-3-snitzer@kernel.org> References: <20261020220346.28020-1-snitzer@kernel.org> <20261020220346.28020-3-snitzer@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ 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1792579992-497039 X-HE-Meta: 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 j8Q5gUAX Hy52wJFlFfHnWco4tu2axfRlJ1gJYnxRd7ZDehLL7lNqaXPcbhsazT3qAJUB1EZmOPi3CavKg7C20MQQbkT79IOkqAN4mxVJbgsbO44XgxUnOvjm+QKt7vMRpYcPIJfZa0S+8oXAKF2/oQLxhYapuh9RZHq07m35Yht/1LwJem7syAxT6wINnV4sRS4oFcMocaFogHcnXa0/hN0nbpfrsQbPgrkJsxrE3uj4qkfMPLASbm/v4xDMZpxqwx3qPuIg27WOryPiwPzZ9G08HdTN6qQ8Yt3GHhuLLh6CvF3PwsCfeO3ItW15i+jPYrZdgfo1qskQJnDenkFePr+wX7a0OMa6FTwfyuS2XpmUNtImWIciNxO7/fKIGrcqkHC8vtNgIJ1fK5v+tY3VjayBcR7JYSPFnh7k8b/SXMlqh8kEjsBDDhReZAmFNHHFEBpAqsx7omcD5qpPaPIGqhAIIJirSCk1znwJSIcbruo29tPjPJ/j/0s4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, 2026-10-20 at 18:03 -0400, Mike Snitzer wrote: > A folio allocated by an RWF_DONTCACHE write is marked dropbehind before > it is dirtied, so folio_account_dirtied() adds it to WB_DONTCACHE_DIRTY > and folio_clear_dirty_for_io() or folio_account_cleaned() subtracts it > when it is cleaned. Nothing marks a folio that already exists. A > caller that sets PG_dropbehind on a folio that is already dirty > unbalances the counter: the clean subtracts what was never added, and > once the counter goes negative, wb_stat_sum() reads it as zero and the > DONTCACHE writeback kick stops writing anything back for every writer > on that bdi_writeback. >=20 > Add folio_mark_dropbehind(), the inverse of the clear in > __filemap_get_folio_mpol(): with the folio locked, mark it and, if it is > dirty, add it to WB_DONTCACHE_DIRTY. The folio lock orders this > against dirtying and against folio_clear_dirty_for_io(). A concurrent > lookup without FGP_DONTCACHE that clears the flag again subtracts what > this added. >=20 > The name follows folio_set_dirty() and folio_mark_dirty(): the first > only sets PG_dirty, the second also accounts the dirty folio. > folio_set_dropbehind() likewise only sets PG_dropbehind, and > folio_mark_dropbehind() also accounts it in WB_DONTCACHE_DIRTY. >=20 > A later commit uses it in NFSD to drop a page shared by two misaligned > WRITEs once both have written it. >=20 > Cc: Jeff Layton > Cc: Jens Axboe > Cc: Matthew Wilcox (Oracle) > Cc: linux-mm@kvack.org > Assisted-by: LLM > Signed-off-by: Mike Snitzer > --- > include/linux/pagemap.h | 1 + > mm/filemap.c | 31 +++++++++++++++++++++++++++++++ > 2 files changed, 32 insertions(+) >=20 > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h > index 0adfa6605653..3012bd95812a 100644 > --- a/include/linux/pagemap.h > +++ b/include/linux/pagemap.h > @@ -1310,6 +1310,7 @@ void end_page_writeback(struct page *page); > void folio_end_writeback(struct folio *folio); > void folio_end_writeback_no_dropbehind(struct folio *folio); > void folio_end_dropbehind(struct folio *folio); > +void folio_mark_dropbehind(struct folio *folio); > void folio_wait_stable(struct folio *folio); > void __folio_mark_dirty(struct folio *folio, struct address_space *, int= warn); > void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb= ); > diff --git a/mm/filemap.c b/mm/filemap.c > index 00fd89cf6f55..ed7e4dbea653 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -1644,6 +1644,37 @@ void folio_end_dropbehind(struct folio *folio) > } > EXPORT_SYMBOL_GPL(folio_end_dropbehind); > =20 > +/** > + * folio_mark_dropbehind - mark an existing folio dropbehind > + * @folio: the folio, locked by the caller > + * > + * Mark @folio so that the writeback that cleans it drops it. A folio t= hat > + * is already dirty is added to WB_DONTCACHE_DIRTY, as folio_account_dir= tied() > + * would have added it had it been marked when dirtied, so that the clea= n > + * that subtracts it leaves the counter balanced. > + */ > +void folio_mark_dropbehind(struct folio *folio) > +{ > + struct address_space *mapping =3D folio->mapping; > + > + VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio); > + > + if (folio_test_dropbehind(folio)) > + return; > + folio_set_dropbehind(folio); > + if (mapping && folio_test_dirty(folio) && > + mapping_can_writeback(mapping)) { > + struct inode *inode =3D mapping->host; > + struct bdi_writeback *wb; > + struct wb_lock_cookie cookie =3D {}; > + > + wb =3D unlocked_inode_to_wb_begin(inode, &cookie); > + wb_stat_mod(wb, WB_DONTCACHE_DIRTY, folio_nr_pages(folio)); > + unlocked_inode_to_wb_end(inode, &cookie); > + } > +} > +EXPORT_SYMBOL_GPL(folio_mark_dropbehind); > + > /** > * folio_end_writeback_no_dropbehind - End writeback against a folio. > * @folio: The folio. Chuck's sashiko had a comment on this one: [Severity: Medium] Could this overcount WB_DONTCACHE_DIRTY if folio_mark_dropbehind() races wi= th filemap_dirty_folio() in mm/page-writeback.c? The dirtying path can set PG_dirty before folio_account_dirtied() checks PG_dropbehind, and it need n= ot hold the folio lock. If folio_mark_dropbehind() sets PG_dropbehind after th= at dirty transition but before the accounting check, both paths increment the counter, while folio_account_cleaned() subtracts it only once. The later NF= SD use of this helper retains the same interleaving. Could the flag update and dirty accounting be serialized so the counter is incremented only once? --=20 Jeff Layton