From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792723555; b=bhTfgaB/NTNHgAKM3fGCCz6PDJ2JCd9gBT5eKZuH0o1NJGS0TX0utubRtJ0fgXpu4PJZ GffFkCx0dUgM9p3KByS2g5mETEXQkcLKIxfEz5MbjWtiTqug8rdamXtkPxCOW6hSG6r+1 sxVA/hwq+nAUDR5HV2yHm4HOhquD+LZgp+5gADZmmUVByhDGs4cTLXvyauNYVS13bOPOd hrhI1T28TVY5g1RQ23YIJMInkkahovvpc40vaDGFWdhaicYAj6gpq0afaIbUdCpWMTsds HQQPFjzLyWM778dJp0DM/PNIuVCTaLojHpDQfCnTy0rqbJWhkMO/z67bBTjcYKfyMXg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792723555; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=FJcW7KrcbrxzeDpcYuSEOOuTzC9YVEEotXFe5+tYCuw=; b=3VE8PBXRsMnTUdjEkcLkMPJcxZJ+INsxCfUb2X/cmLdnw7REJaWiFYh7BfJwciuFtdb3 Vi/BT2mgzS2pbjE/bpbEiyQ4jN3bkGGzWvavkeMWq9HSbgGfbpfE8P32wybFgjVQ57aVS YGx7ItiJ4jwSsmCuIDxryMkYzBeR6y6xC/hnUb8sthVrQhpZ79U6Y++RAY/SeWsfpb24T bkS9iHh0wWP4It4IsTZRYgKVvIKOB+er0CALbg6uqKqmI+kQh1fwkA9w54VFK2WiQQTPd HI/CIIjlWx6t/GX+fAdaOw/DVRH/ZIi3a9WVXPOR3VcIr4ZSBj5y5tpzN4RVRsEHjJA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=WKPlSc4e; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 99FDD606A320; Fri, 23 Oct 2026 02:45:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=WKPlSc4e Received: by smtp.kernel.org (Postfix) with ESMTPSA id 088FD1F01828; Fri, 23 Oct 2026 02:45:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792723555; bh=FJcW7KrcbrxzeDpcYuSEOOuTzC9YVEEotXFe5+tYCuw=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=WKPlSc4eioB0oO3aBYZ34PcGIQYEKGjZPiCkmZXHv0ybz0jpmEUuR3XoYr2iluB3/ xpWsohlCYSIuZuJJ60AwyyLO/vlOVrHugMBonjkv+8u9y7o84ecGANPK8r9zEgkWG/ YkT8DJyRRDNCVuHZuIFHqoTeZ7IpurQKnCLem/22qTeiSAHCVRL8wHJmskNh71qzz+ hB4wKkHPX9fIaOHvOlVl9hU7KU/2WKFfamVZ2KcuHE+71TAgVURbzJq/AtDcu1Do3j dT/c7AQonI7+AlDhZ6t0Gibipt29hWGfnIoSk+InLrQptQrn9niZhUcnIeB/GJZPjH ANRPELDTH4Z7w== From: Mike Snitzer To: Chuck Lever , Jeff Layton 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 Subject: [PATCH v8 4/7] mm: add folio_mark_dropbehind() to mark a folio dropbehind and account it Date: Thu, 22 Oct 2026 22:45:45 -0400 Message-ID: <20261023024548.71123-5-snitzer@kernel.org> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20261023024548.71123-1-snitzer@kernel.org> References: <20261023024548.71123-1-snitzer@kernel.org> X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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. Add folio_mark_dropbehind(), the inverse of the clear in __filemap_get_folio_mpol(): with the folio locked, mark it and, if it has been counted as dirty, add it to WB_DONTCACHE_DIRTY. PG_dirty is not the test for that: filemap_dirty_folio() sets it before folio_account_dirtied() counts the folio, under the i_pages lock and without the folio lock, so a folio marked between the two would be counted twice. folio_account_dirtied() sets the folio's dirty tag in the same i_pages section that counts it, and every path that uncounts a dirty folio holds the folio lock, so set the flag and test the dirty tag under the i_pages lock: a dirtier not yet counting the folio finds the flag set and counts it itself. Under that lock the wb is inode_to_wb(), as in folio_account_dirtied(). A concurrent lookup without FGP_DONTCACHE that clears the flag again subtracts what this added. 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. A later commit uses it in NFSD to drop a page shared by two misaligned WRITEs once both have written it. Signed-off-by: Mike Snitzer --- include/linux/pagemap.h | 1 + mm/filemap.c | 39 +++++++++++++++++++++++++++++++++++++++ 2 files changed, 40 insertions(+) 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 1cc28224d547..0c76ee5750b3 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1644,6 +1644,45 @@ void folio_end_dropbehind(struct folio *folio) } EXPORT_SYMBOL_GPL(folio_end_dropbehind); +/** + * 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 + * already counted as dirty is added to WB_DONTCACHE_DIRTY, as + * folio_account_dirtied() would have added it had it been marked when + * dirtied, so that the clean that subtracts it leaves the counter balanced. + * + * folio_account_dirtied() counts a folio under the i_pages lock, in the + * section that sets its dirty tag, after PG_dirty has been set without the + * folio lock, and every path that uncounts a dirty folio holds the folio + * lock. So with the folio locked and the i_pages lock held, the dirty tag, + * not PG_dirty, says whether the folio has been counted: a dirtier past + * folio_test_set_dirty() but not yet counting finds the flag set and adds + * the folio itself. + */ +void folio_mark_dropbehind(struct folio *folio) +{ + struct address_space *mapping = folio->mapping; + unsigned long flags; + + VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio); + + if (folio_test_dropbehind(folio)) + return; + if (!mapping || !mapping_can_writeback(mapping)) { + folio_set_dropbehind(folio); + return; + } + xa_lock_irqsave(&mapping->i_pages, flags); + folio_set_dropbehind(folio); + if (xa_get_mark(&mapping->i_pages, folio->index, PAGECACHE_TAG_DIRTY)) + wb_stat_mod(inode_to_wb(mapping->host), WB_DONTCACHE_DIRTY, + folio_nr_pages(folio)); + xa_unlock_irqrestore(&mapping->i_pages, flags); +} +EXPORT_SYMBOL_GPL(folio_mark_dropbehind); + /** * folio_end_writeback_no_dropbehind - End writeback against a folio. * @folio: The folio. -- 2.52.0