On Fri, Mar 15, 2013 at 06:28:18PM -0400, Eric Whitney wrote:
> I'm seeing Xfstest 285 consistently fail for the 1k test case using the
> latest dev branch while running on both x86 and ARM. Subtest 08 is
> the problem. From the test output:
>
> 08. Test file with unwritten extents, only have unwritten pages
> 08.01 SEEK_HOLE expected 0 or 4194304, got 11264. FAIL
> 08.02 SEEK_HOLE expected 1 or 4194304, got 11264. FAIL
> 08.03 SEEK_DATA expected 10240 or 10240, got 0. FAIL
> 08.04 SEEK_DATA expected 10240 or 10240, got 1. FAIL
>
> From previous discussions, we expect 285 to fail in the ext3 (nodelalloc,
> no flex_bg, and no extents) test case, but in subtest 07. It still does
> that.
>
> In the dev branch, reverting 4f42f80a8f - "ext4: use s_extent_max_zeroout_kb
> value as number of kb" - results in success for 285 in the 1k test case.
Hi Eric,
I see what's going on. First of all it isn't a bug. :-) Please let me
describe why it happens.
In this commit (4f42f80a8f), it tries to fix a bug that we never zero
out an unwritten extent. So after applied it, when an unwritten extent
is converted, it could be zeroed out. In xfstests #285 subtest 08 it
preallocates an unwritten extent which is 4MB. Then it writes some data
at offset 10 * blocksize, which the length is one blocksize, and calles
sync_file_range(2) to flush it. So the call trace looks like:
ext4_fallocate()
->ext4_map_blocks()
[one unwritten extent is allocated]
ext4_file_write()
ext4_da_writepages()
->ext4_map_blocks() with EXT4_GET_BLOCKS_CREATE flag
->ext4_ext_handle_uninitialized_extents()
->ext4_ext_convert_to_initialized()
In ext4_ext_convert_to_initialized() it tries to zero out unwritten
extent if condition is matched. Let's see what happens.
case a) 1k block size
max_zeroout: 32
ee_len: 4096
allocated: 4086
m_len: 1
In this case, the following condition is matched.
fs/ext4/extents.c:3310
else if (map->m_lblk - ee_block + map-m_len < max_zeroout)
10 - 0 + 1 < 32
So unwritten extent [0,11] will be converted to written. That is why
11264 (11 * 1k) is returned when we seek a hole from offset 0 and 1,
and 0 and 1 are returned when we seek a data from offset 0 and 1.
case b) 4k block size
max_zeroout: 8
ee_len: 1024
allocated: 1014
m_len: 1
In this case, the above condition won't be matched.
else if (map->m_lblk - ee_block + map-m_len < max_zeroout)
10 - 0 + 1 < 8
So only one unwritten extent [10, 1] is converted, and the test can
pass.
Regards
- Zheng
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