Re: [PATCH] xfs: btree format inode forks can have zero extents

[Chandan Babu R <chandanrlinux@gmail.com>]

On 27 May 2021 at 05:49, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> xfs/538 is assert failing with this trace when testing with
> directory block sizes of 64kB:
>
> XFS: Assertion failed: !xfs_need_iread_extents(ifp), file: fs/xfs/libxfs/xfs_bmap.c, line: 608
> ....
> Call Trace:
>  xfs_bmap_btree_to_extents+0x2a9/0x470
>  ? kmem_cache_alloc+0xe7/0x220
>  __xfs_bunmapi+0x4ca/0xdf0
>  xfs_bunmapi+0x1a/0x30
>  xfs_dir2_shrink_inode+0x71/0x210
>  xfs_dir2_block_to_sf+0x2ae/0x410
>  xfs_dir2_block_removename+0x21a/0x280
>  xfs_dir_removename+0x195/0x1d0
>  xfs_remove+0x244/0x460
>  xfs_vn_unlink+0x53/0xa0
>  ? selinux_inode_unlink+0x13/0x20
>  vfs_unlink+0x117/0x220
>  do_unlinkat+0x1a2/0x2d0
>  __x64_sys_unlink+0x42/0x60
>  do_syscall_64+0x3a/0x70
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> This is a check to ensure that the extents have been read into
> memory before we are doing a ifork btree manipulation. This assert
> is bogus in the above case.
>
> We have a fragmented directory block that has more extents in it
> than can fit in extent format, so the inode data fork is in btree
> format. xfs_dir2_shrink_inode() asks to remove all remaining 16
> filesystem blocks from the inode so it can convert to short form,
> and __xfs_bunmapi() removes all the extents. We now have a data fork
> in btree format but have zero extents in the fork. This incorrectly
> trips the xfs_need_iread_extents() assert because it assumes that an
> empty extent btree means the extent tree has not been read into
> memory yet. This is clearly not the case with xfs_bunmapi(), as it
> has an explicit call to xfs_iread_extents() in it to pull the
> extents into memory before it starts unmapping.
>
> Also, the assert directly after this bogus one is:
>
> 	ASSERT(ifp->if_format == XFS_DINODE_FMT_BTREE);
>
> Which covers the context in which it is legal to call
> xfs_bmap_btree_to_extents just fine. Hence we should just remove the
> bogus assert as it is clearly wrong and causes a regression.
>
> The returns the test behaviour to the pre-existing assert failure in
> xfs_dir2_shrink_inode() that indicates xfs_bunmapi() has failed to
> remove all the extents in the range it was asked to unmap.
>

The functions calling xfs_bmap_btree_to_extents() have indeed read all the
extents of the corresponding inode fork into memory. Hence, removal of the
assert() statement is not an issue.

Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

> Signed-off-by: Dave Chinner <dchinner@redhat.com>

--
chandan