### question
We recently used fuzz(hydra) to test 4.19 stable XFS and automatically generate tmp.img (XFS v5 format, but some metadata is wrong)
Test as follows:
mount tmp.img tmpdir
cp file tmpdir
sync --> stuck
### cause analysis
This is because tmp.img (only 1 AG) has some problems. Using xfs_repair detect information as follows:
agf_freeblks 0, counted 3224 in ag 0
agf_longest 536874136, counted 3224 in ag 0
sb_fdblocks 613, counted 3228
The reason sync is blocked is :
xfs_vm_writepages(xfs_address_space_operations--writepages)
write_cache_pages
xfs_do_writepage
xfs_writepage_map
xfs_map_blocks
allocate_blocks:
error = xfs_iomap_write_allocate
xfs_iomap_write_allocate
while (count_fsb != 0) {
nimaps = 0;
while (nimaps == 0) { --> endless loop
nimaps = 1;
error = xfs_bmapi_write(..., &nimaps) --> nimaps becomes 0 again
xfs_bmapi_write
xfs_bmap_alloc
xfs_bmap_btalloc
xfs_alloc_vextent
xfs_alloc_fix_freelist
xfs_alloc_space_available --> less space than needed
xfs_alloc_space_available
alloc_len = args->minlen + (args->alignment - 1) + args->minalignslop;
longest = xfs_alloc_longest_free_extent(pag, min_free, reservation);
if (longest < alloc_len)
return false;
/* do we have enough free space remaining for the allocation? */
available = (int)(pag->pagf_freeblks + pag->pagf_flcount -
reservation - min_free - args->minleft);
if (available < (int)max(args->total, alloc_len))
return false;
### solve
1. Detect the above metadata corruption when mounting XFS?
agf_freeblks 0, counted 3224 in ag 0
agf_longest 536874136, counted 3224 in ag 0
sb_fdblocks 613, counted 3228
2. xfs_repair detection at system boot? If xfs_repair fails, refuse to mount XFS
