From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E7B4BC3525A for ; Wed, 26 Jan 2022 00:59:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235469AbiAZA7L (ORCPT ); Tue, 25 Jan 2022 19:59:11 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:51892 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234910AbiAZA7K (ORCPT ); Tue, 25 Jan 2022 19:59:10 -0500 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 0C2F621122 for ; Wed, 26 Jan 2022 00:59:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1643158749; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=vxuxrwFizSCtowF8HPPpCs6xQ5QeSd0VtpH6bXDH/eg=; b=AXtRESNcamJosTIhlYFKPZLz1VDUcvsaD1S7YvQH36TkarE3PnMDFTiawWDqkGGBCutHrY 4S3Jeg8124Y6sQoL1VzAGDH+gbf4AKQa5yudpLuQ4bYjT0Dn+Qodf+EFfDDlXv4ovk8dmL yWrp5iftmICAAqb9MHSZTxvrTScD+0E= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 5E338133F5 for ; Wed, 26 Jan 2022 00:59:08 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id doEgCtyc8GH7BwAAMHmgww (envelope-from ) for ; Wed, 26 Jan 2022 00:59:08 +0000 From: Qu Wenruo To: linux-btrfs@vger.kernel.org Subject: [PATCH v3 1/3] btrfs: defrag: don't try to merge regular extents with preallocated extents Date: Wed, 26 Jan 2022 08:58:48 +0800 Message-Id: <20220126005850.14729-1-wqu@suse.com> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org [BUG] With older kernels (before v5.16), btrfs will defrag preallocated extents. While with newer kernels (v5.16 and newer) btrfs will not defrag preallocated extents, but it will defrag the extent just before the preallocated extent, even it's just a single sector. This can be exposed by the following small script: mkfs.btrfs -f $dev > /dev/null mount $dev $mnt xfs_io -f -c "pwrite 0 4k" -c sync -c "falloc 4k 16K" $mnt/file xfs_io -c "fiemap -v" $mnt/file btrfs fi defrag $mnt/file sync xfs_io -c "fiemap -v" $mnt/file The output looks like this on older kernels: /mnt/btrfs/file: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 26624..26631 8 0x0 1: [8..39]: 26632..26663 32 0x801 /mnt/btrfs/file: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..39]: 26664..26703 40 0x1 Which defrags the single sector along with the preallocated extent, and replace them with an regular extent into a new location (caused by data COW). This wastes most of the data IO just for the preallocated range. On the other hand, v5.16 is slightly better: /mnt/btrfs/file: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 26624..26631 8 0x0 1: [8..39]: 26632..26663 32 0x801 /mnt/btrfs/file: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 26664..26671 8 0x0 1: [8..39]: 26632..26663 32 0x801 The preallocated range is not defragged, but the sector before it still gets defragged, which has no need for it. [CAUSE] One of the function reused by the old and new behavior is defrag_check_next_extent(), it will determine if we should defrag current extent by checking the next one. It only checks if the next extent is a hole or inlined, but it doesn't check if it's preallocated. On the other hand, out of the function, both old and new kernel will reject preallocated extents. Such inconsistent behavior causes above behavior. [FIX] - Also check if next extent is preallocated If so, don't defrag current extent. - Add comments for each branch why we reject the extent This will reduce the IO caused by defrag ioctl and autodefrag. Signed-off-by: Qu Wenruo --- Changelog: v2: - Use @extent_thresh from caller to replace the harded coded threshold Now caller has full control over the extent threshold value. - Remove the old ambiguous check based on physical address The original check is too specific, only reject extents which are physically adjacent, AND too large. Since we have correct size check now, and the physically adjacent check is not always a win. So remove the old check completely. v3: - Split the @extent_thresh and physicall adjacent check into other patches - Simplify the comment --- fs/btrfs/ioctl.c | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-) diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c index 91ba2efe9792..0d8bfc716e6b 100644 --- a/fs/btrfs/ioctl.c +++ b/fs/btrfs/ioctl.c @@ -1053,19 +1053,25 @@ static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em, bool locked) { struct extent_map *next; - bool ret = true; + bool ret = false; /* this is the last extent */ if (em->start + em->len >= i_size_read(inode)) - return false; + return ret; next = defrag_lookup_extent(inode, em->start + em->len, locked); + /* No more em or hole */ if (!next || next->block_start >= EXTENT_MAP_LAST_BYTE) - ret = false; - else if ((em->block_start + em->block_len == next->block_start) && - (em->block_len > SZ_128K && next->block_len > SZ_128K)) - ret = false; - + goto out; + /* Preallocated */ + if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) + goto out; + /* Physically adjacent and large enough */ + if ((em->block_start + em->block_len == next->block_start) && + (em->block_len > SZ_128K && next->block_len > SZ_128K)) + goto out; + ret = true; +out: free_extent_map(next); return ret; } -- 2.34.1