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

[Darrick J. Wong]

[add Christoph to cc since he added/last touched this assert, I think]

On Mon, Mar 27, 2023 at 10:02:18PM +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

Why not get rid of the assertion?  It's not like it changes the course
of the code flow -- userspace still gets told there's a delalloc extent.

Or, if the assert does serve some purpose, then do we need to take
the mmaplock for cow fork reporting too?

--D

> ------------[ 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, hold XFS_MMAPLOCK_EXCL lock when do xfs_getbmap(),
> to prevent write operations by do_page_mkwrite().
> During doing __xfs_filemap_fault() we can't hold IOLOCK lock, as it's may lead
> to ABBA dealock with xfs_file_write_iter().It's very easy to reproduce when
> do fsstress, lockdep will detect deadlock.
> 
> Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_bmap_util.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index a09dd2606479..f23771a0cc8d 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -463,11 +463,13 @@ xfs_getbmap(
>  			max_len = XFS_ISIZE(ip);
>  		break;
>  	case XFS_DATA_FORK:
> +		lock = XFS_MMAPLOCK_EXCL;
> +		xfs_ilock(ip, lock);
>  		if (!(iflags & BMV_IF_DELALLOC) &&
>  		    (ip->i_delayed_blks || XFS_ISIZE(ip) > ip->i_disk_size)) {
>  			error = filemap_write_and_wait(VFS_I(ip)->i_mapping);
>  			if (error)
> -				goto out_unlock_iolock;
> +				goto out_unlock_ilock;
>  
>  			/*
>  			 * Even after flushing the inode, there can still be
> @@ -486,7 +488,7 @@ xfs_getbmap(
>  		else
>  			max_len = XFS_ISIZE(ip);
>  
> -		lock = xfs_ilock_data_map_shared(ip);
> +		lock |= xfs_ilock_data_map_shared(ip);
>  		break;
>  	}
>  
> -- 
> 2.31.1
>