Re: [PATCH v3] xfs: fix BUG_ON in xfs_getbmap()

[Darrick J. Wong]

On Wed, Mar 29, 2023 at 10:52:58AM +0800, Ye Bin wrote:
> From: Ye Bin <yebin10@xxxxxxxxxx>
> 
> There's issue as follows:
> XFS: Assertion failed: (bmv->bmv_iflags & BMV_IF_DELALLOC) != 0, file: fs/xfs/xfs_bmap_util.c, line: 329
> ------------[ cut here ]------------
> kernel BUG at fs/xfs/xfs_message.c:102!
> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> CPU: 1 PID: 14612 Comm: xfs_io Not tainted 6.3.0-rc2-next-20230315-00006-g2729d23ddb3b-dirty #422
> RIP: 0010:assfail+0x96/0xa0
> RSP: 0018:ffffc9000fa178c0 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: 0000000000000001 RCX: ffff888179a18000
> RDX: 0000000000000000 RSI: ffff888179a18000 RDI: 0000000000000002
> RBP: 0000000000000000 R08: ffffffff8321aab6 R09: 0000000000000000
> R10: 0000000000000001 R11: ffffed1105f85139 R12: ffffffff8aacc4c0
> R13: 0000000000000149 R14: ffff888269f58000 R15: 000000000000000c
> FS:  00007f42f27a4740(0000) GS:ffff88882fc00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000b92388 CR3: 000000024f006000 CR4: 00000000000006e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  xfs_getbmap+0x1a5b/0x1e40
>  xfs_ioc_getbmap+0x1fd/0x5b0
>  xfs_file_ioctl+0x2cb/0x1d50
>  __x64_sys_ioctl+0x197/0x210
>  do_syscall_64+0x39/0xb0
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> Above issue may happen as follows:
>          ThreadA                       ThreadB
> do_shared_fault
>  __do_fault
>   xfs_filemap_fault
>    __xfs_filemap_fault
>     filemap_fault
>                              xfs_ioc_getbmap -> Without BMV_IF_DELALLOC flag
> 			      xfs_getbmap
> 			       xfs_ilock(ip, XFS_IOLOCK_SHARED);
> 			       filemap_write_and_wait
>  do_page_mkwrite
>   xfs_filemap_page_mkwrite
>    __xfs_filemap_fault
>     xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
>     iomap_page_mkwrite
>      ...
>      xfs_buffered_write_iomap_begin
>       xfs_bmapi_reserve_delalloc -> Allocate delay extent
>                               xfs_ilock_data_map_shared(ip)
> 	                      xfs_getbmap_report_one
> 			       ASSERT((bmv->bmv_iflags & BMV_IF_DELALLOC) != 0)
> 	                        -> trigger BUG_ON
> 
> As xfs_filemap_page_mkwrite() only hold XFS_MMAPLOCK_SHARED lock, there's
> small window mkwrite can produce delay extent after file write in xfs_getbmap().
> To solve above issue, just skip delalloc extents.
> 
> Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_bmap_util.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index a09dd2606479..f032d3a4b727 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -314,15 +314,13 @@ xfs_getbmap_report_one(
>  	if (isnullstartblock(got->br_startblock) ||
>  	    got->br_startblock == DELAYSTARTBLOCK) {
>  		/*
> -		 * Delalloc extents that start beyond EOF can occur due to
> -		 * speculative EOF allocation when the delalloc extent is larger
> -		 * than the largest freespace extent at conversion time.  These
> -		 * extents cannot be converted by data writeback, so can exist
> -		 * here even if we are not supposed to be finding delalloc
> -		 * extents.
> +		 * Take the flush completion as being a point-in-time snapshot
> +		 * where there are no delalloc extents, and if any new ones
> +		 * have been created racily, just skip them as being 'after'
> +		 * the flush and so don't get reported.
>  		 */
> -		if (got->br_startoff < XFS_B_TO_FSB(ip->i_mount, XFS_ISIZE(ip)))
> -			ASSERT((bmv->bmv_iflags & BMV_IF_DELALLOC) != 0);
> +		if (!(bmv->bmv_iflags & BMV_IF_DELALLOC))
> +			return 0;

I think I'm ok with this...
Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>

--D

>  
>  		p->bmv_oflags |= BMV_OF_DELALLOC;
>  		p->bmv_block = -2;
> -- 
> 2.31.1
>