From: Theodore Ts'o Subject: [PATCH 18/28] ext4: Fix fencepost error in chosing choosing group vs file preallocation. Date: Tue, 2 Mar 2010 13:18:35 -0500 Message-ID: <1267553925-6308-19-git-send-email-tytso@mit.edu> References: <1267553925-6308-1-git-send-email-tytso@mit.edu> Cc: Tao Ma , "Theodore Ts'o" To: Ext4 Developers List Return-path: Received: from THUNK.ORG ([69.25.196.29]:43061 "EHLO thunker.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753402Ab0CBSS4 (ORCPT ); Tue, 2 Mar 2010 13:18:56 -0500 In-Reply-To: <1267553925-6308-1-git-send-email-tytso@mit.edu> Sender: linux-ext4-owner@vger.kernel.org List-ID: From: Tao Ma The ext4 multiblock allocator decides whether to use group or file preallocation based on the file size. When the file size reaches s_mb_stream_request (default is 16 blocks), it changes to use a file-specific preallocation. This is cool, but it has a tiny problem. See a simple script: mkfs.ext4 -b 1024 /dev/sda8 1000000 mount -t ext4 -o nodelalloc /dev/sda8 /mnt/ext4 for((i=0;i<5;i++)) do cat /mnt/4096>>/mnt/ext4/a #4096 is a file with 4096 characters. cat /mnt/4096>>/mnt/ext4/b done debuge4fs -R 'stat a' /dev/sda8|grep BLOCKS -A 1 And you get BLOCKS: (0-14):8705-8719, (15):2356, (16-19):8465-8468 So there are 3 extents, a bit strange for the lonely 15th logical block. As we write to the 16 blocks, we choose file preallocation in ext4_mb_group_or_file, but in ext4_mb_normalize_request, we meet with the 16*1024 range, so no preallocation will be carried. file b then reserves the space after '2356', so when when write 16, we start from another part. This patch just change the check in ext4_mb_group_or_file, so that for the lonely 15 we will still use group preallocation. After the patch, we will get: debuge4fs -R 'stat a' /dev/sda8|grep BLOCKS -A 1 BLOCKS: (0-15):8705-8720, (16-19):8465-8468 Looks more sane. Thanks. Signed-off-by: Tao Ma Signed-off-by: "Theodore Ts'o" --- fs/ext4/mballoc.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-) diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index 415e11f..72d5ceb 100644 --- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -3935,7 +3935,7 @@ static void ext4_mb_group_or_file(struct ext4_allocation_context *ac) /* don't use group allocation for large files */ size = max(size, isize); - if (size >= sbi->s_mb_stream_request) { + if (size > sbi->s_mb_stream_request) { ac->ac_flags |= EXT4_MB_STREAM_ALLOC; return; } -- 1.6.6.1.1.g974db.dirty