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ext4: Fix fencepost error in chosing choosing group vs file preallocation.
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 <tao.ma@oracle.com> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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@ -3935,7 +3935,7 @@ static void ext4_mb_group_or_file(struct ext4_allocation_context *ac)
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/* don't use group allocation for large files */
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size = max(size, isize);
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if (size >= sbi->s_mb_stream_request) {
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if (size > sbi->s_mb_stream_request) {
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ac->ac_flags |= EXT4_MB_STREAM_ALLOC;
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return;
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}
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