Hi,
I am Jayashree Mohan, a PhD student at the University of Texas at
Austin, working with Prof. Vijay Chidambaram. I've been working on the
Crashmonkey project, which is a test harness for file system crash
consistency checks[1].
While trying to reproduce some of the reported bugs on filesystems
using CrashMonkey we stumbled upon the following issue:
Whenever we have a collapse_range operation that follows fallocate
with keep_size that allocated blocks beyond the end-of-file, the eof
blocks are lost due to the collapse_range operation in f2fs and xfs.
Consider the following workload(The number within the braces indicate
the start and end offsets)
1. Write (0 - 428K)
Sync()
2. Fallocate keep_size (428K - 432K )
3. Fallocate collapse_range (112K - 128K)
If we now stat the file, the result is as follows for ext4:
1. After write (0-428K)
Size: 438272 Blocks: 856 IO Block: 1024 regular file
2. Falloc keep_size 428K - 432K
Size: 438272 Blocks: 864 IO Block: 1024 regular file
3. Falloc collapse_range 112K - 128K
Size: 421888 Blocks: 832 IO Block: 1024 regular file
The block count at the end of collapse_range is 32 blocks less than
step 2, which corresponds to the 16K range that was collapsed. So this
behavior makes sense. However consider the same for f2fs and xfs,
If we now stat the file, the result is as follows for f2fs:
1. After write (0-428K)
Size: 438272 Blocks: 856 IO Block: 4096 regular file
2. Falloc keep_size 428K - 432K
Size: 438272 Blocks: 864 IO Block: 4096 regular file
3. Falloc collapse_range 112K - 128K
Size: 421888 Blocks: 824 IO Block: 4096 regular file
If we now stat the file, the result is as follows for xfs:
1. After write (0-428K)
Size: 438272 Blocks: 856 IO Block: 4096 regular file
xfs_bmap : 0: [0..855]: 160..1015
2. Falloc keep_size 428K - 432K
Size: 438272 Blocks: 864 IO Block: 4096 regular file
xfs_bmap : 0: [0..855]: 160..1015
1: [856..863]: 1016..1023
3. Falloc collapse_range 112K - 128K
Size: 421888 Blocks: 824 IO Block: 4096 regular file
xfs_bmap : 0: [0..223]: 160..383
1: [224..823]: 416..1015
If we didn't have the keep_size option with fallocate in step 2, then
we see the xfs_bmap after step 3 to be
0: [0..223]: 160..383
1: [224..823]: 416..1015
2: [824..831]: 1016..1023
This clearly shows, the blocks allocated beyond the eof are lost
during the collapse_range operation, if keep_size was specified while
allocating these blocks. This behavior is seen in both f2fs and xfs,
while ext4 seems to preserve the allocated blocks. All these were
tested on linux kernel 4.15. Shouldn't f2fs and xfs also behave like
ext4?
Let me know if I am missing some detail.
[1] https://github.com/utsaslab/crashmonkey
Thanks,
Jayashree
