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=1793740681; b=FczNtmaDxU6mJ/DohrhRGXM+R/xbea0O6wl3mcDNLntQyxCTfLABVZbIkfuBRDajcANf qGMnZ9VpdHRFnBbyMUioHa1bu4BVlxtFc+xZJYFDAX/hTXG77ZvDHOBXyEAAhbIOcplTu 9OqLiasP+qhzvUsTVAWuOg+6x4ZKxKqW8rxpx93+cI18SP1cMThPAg9Uewqj9KF7UQX4w 7cdN+yPV1oWR/diEkrNLEX1xeiUUWeZzlzj03nipDQ/JUen2TZsB3MWHmkwTMazBlCqn4 CZvphc2fjl/C2vEgoWSIHqlxXCC5JNA43PMl990of312ChgtOjsps4zanHyB+HcpB6Q== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793740681; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=DI89KFBg08cV0NMoZcQe793KHlTmMZi7dlrFBwSyZaw=; b=x4PzAF40GYmR9wgDFgpJ4Cj5ITbTzqjBZkrLcSBG70tcrHv1tX7kAdg0MLBAllYyVLri zNVbHe8iuV9O2wNhnM7YY5YWz5jrvSvPoF0g03+8dahdSaIv9f1EpKmuXnmaBWafCPLWq VyOffbHqjNwuXC/gCzF2FGaNwVkINz1pdhvG6QxYuBO5lzZGOyNMQhbIHz3oADf+UUdM8 72R+LPZxo2OZfdXcOe1w83XZ6Nv0NE4el8CQ13vNoB//ORu9SDDsWOMGt43zeZj3COATD a9DxQNodjuQSPpv7cajMG3UH3YpaUnLs082IfoKuVGK9BbwCwhIZ8Ea0P5pMm3MgUFQ== 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=aUSE2kt3; 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)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F0164606A334; Tue, 3 Nov 2026 21:18:00 +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=aUSE2kt3 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 738351F00A1A; Tue, 3 Nov 2026 21:18:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793740680; bh=DI89KFBg08cV0NMoZcQe793KHlTmMZi7dlrFBwSyZaw=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=aUSE2kt3f6UaFdPxcLV4ZJIDjVzGjYBMNsGVkFaf1jo9LkWqkjrz4yl4GpX3IBkTw zIG7m/fXJrWMSoMHe9SSopQykRMeM6LMaz55t+/MiJj59pPWz5CaTQEEj4XQ4bpziT F+/t8qLzhHwcFXI8UlXRZhEFGmUFPGd+rLAGri/sx+ni91puKPc3WwkuMoaG1Rrwt0 RdtCnNy0Wj8Lnmw/tsSI5hd+T40vgO/UWTIKJ78LTF2A/YP0dCnbkWkLSBKJkuXv8/ /5dmqlkK76mQyL7x72g9ixA1r8WLpTD3pq2tzfCeFl5gr6iwl3abwQbWHWcBz2wFSK oZOpn5JeHMc2g== From: Mike Snitzer To: Matthew Wilcox , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org Cc: Chuck Lever , Jeff Layton , linux-nfs@vger.kernel.org, NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Christoph Hellwig , Jens Axboe , Jonathan Corbet Subject: [PATCH v10 4/7] mm: add folio_mark_dropbehind() to mark a folio dropbehind and account it Date: Tue, 3 Nov 2026 16:17:51 -0500 Message-ID: <20261103211754.35352-5-snitzer@kernel.org> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20261103211754.35352-1-snitzer@kernel.org> References: <20261103211754.35352-1-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 WB_DONTCACHE_DIRTY counts the dirty folios that are marked dropbehind. When a dropbehind folio is dirtied, folio_account_dirtied() adds it to the counter. When the folio is cleaned, folio_clear_dirty_for_io() or folio_account_cleaned() subtracts it again. This balances only if the folio is marked before it is dirtied, as it is for the folios that an RWF_DONTCACHE write creates. To prepare for NFSD to mark a folio dropbehind once both writes that share it have written their parts, handle a folio that is already dirty when it is marked. folio_set_dropbehind() only sets the flag. With it, the clean would subtract the folio from WB_DONTCACHE_DIRTY although it was never added, and the counter would go negative. Add folio_mark_dropbehind(), the inverse of filemap_clear_dropbehind(). With the folio locked, it sets PG_dropbehind. Under the i_pages lock, it adds the folio to WB_DONTCACHE_DIRTY if both PG_dirty and the dirty tag are set. A dirtier that has set PG_dirty but not yet counted the folio finds the flag set and counts the folio itself. The name follows folio_set_dirty() and folio_mark_dirty(): the set variant only sets the flag, the mark variant also accounts for it. Reviewed-by: Jeff Layton Signed-off-by: Mike Snitzer --- Notes: Notes on questions from earlier reviews, with the code that answers each: - Counting rule. The same rule as filemap_clear_dropbehind() in 2/7: folio_mark_dropbehind() counts the folio only when folio_test_dirty() and PAGECACHE_TAG_DIRTY are both true. The test runs under the folio lock (VM_WARN_ON_ONCE_FOLIO checks it) and the i_pages lock. A stale tag on a clean folio adds no count. - Double count. A folio that has PG_dropbehind returns at the first test, before it changes the count. - No writeback. A mapping without writeback gets only the flag. folio_account_dirtied() does not count such a folio either (mapping_can_writeback()). - Known window. As in 2/7: a stale tag together with an unlocked dirtier that has set PG_dirty but not yet taken the i_pages lock can make this count a folio that the dirtier then counts again. Only the counter is affected. include/linux/pagemap.h | 1 + mm/filemap.c | 32 ++++++++++++++++++++++++++++++++ 2 files changed, 33 insertions(+) diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 73af18a373670..3f5295d6fd73b 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -1306,6 +1306,7 @@ 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 3a13289c07dc4..5134227ba3a23 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1643,6 +1643,38 @@ 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. If the folio + * is already counted as dirty, which with the i_pages lock held means both + * PG_dirty and the dirty tag are set, add it to WB_DONTCACHE_DIRTY so that + * the clean that subtracts it leaves the counter balanced. + */ +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 (folio_test_dirty(folio) && + 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