Re: [PATCH] xfs: reserve blocks for rmapbt changes in xfs_reflink_end_cow

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Nov 26, 2018 at 09:44:56AM -0500, Brian Foster wrote:
> On Fri, Nov 23, 2018 at 09:54:48AM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > In xfs_reflink_end_cow, we have to swap written extents from the CoW
> > fork into the data fork, which can require extensive rmapbt updates.
> > The transaction block reservation calculation forgot that part of the
> > calculation, which lead to a shutdown during an end_cow transaction roll
> > during fsx exercises:
> > 
> > XFS: Assertion failed: tp->t_blk_res >= tp->t_blk_res_used, file: fs/xfs/xfs_trans.c, line: 116
> > <machine registers snipped>
> > Call Trace:
> >  xfs_trans_dup+0x211/0x250 [xfs]
> >  xfs_trans_roll+0x6d/0x180 [xfs]
> >  xfs_defer_trans_roll+0x10c/0x3b0 [xfs]
> >  xfs_defer_finish_noroll+0xdf/0x740 [xfs]
> >  xfs_defer_finish+0x13/0x70 [xfs]
> >  xfs_reflink_end_cow+0x2c6/0x680 [xfs]
> >  xfs_dio_write_end_io+0x115/0x220 [xfs]
> >  iomap_dio_complete+0x3f/0x130
> >  iomap_dio_rw+0x3c3/0x420
> >  xfs_file_dio_aio_write+0x132/0x3c0 [xfs]
> >  xfs_file_write_iter+0x8b/0xc0 [xfs]
> >  __vfs_write+0x193/0x1f0
> >  vfs_write+0xba/0x1c0
> >  ksys_write+0x52/0xc0
> >  do_syscall_64+0x50/0x160
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> 
> It's a bit interesting that we only seem to use XFS_NRMAPADD_SPACE_RES()
> in XFS_SWAP_RMAP_SPACE_RES(), and then the latter (more expectedly) is
> only used in the swap extent operation. Any particular reason for that?
> IOW, we don't seem to include this res in places where we do extent
> allocs and whatnot, which also (defer) rmap updates..

<scrubs all the cobwebs out of his brain>

Normally the per-AG reservation is supposed to handle expansions of the
rmap and refcount btrees, so I think this patch isn't correct.

OTOH, looking again at the code, I see...

	offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
	end_fsb = XFS_B_TO_FSB(ip->i_mount, offset + count);

	resblks = XFS_NEXTENTADD_SPACE_RES(ip->i_mount,
			(unsigned int)(end_fsb - offset_fsb),
			XFS_DATA_FORK);

So if, say, blocksize = 4096, offset = 512 and count = 1024, then
offset_fsb = 0 and end_fsb = 2, so this reserves enough blocks for
swapping 2 - 0 blocks, whereas the range covers three different blocks.

Hmm, I guess I'll try that, though the overflow took a while to hit. :)

--D

> Brian
> 
> >  fs/xfs/xfs_reflink.c |   11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > index 322a852ce284..c706d7791479 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -663,9 +663,14 @@ xfs_reflink_end_cow(
> >  		ASSERT(0);
> >  		goto out;
> >  	}
> > -	resblks = XFS_NEXTENTADD_SPACE_RES(ip->i_mount,
> > -			(unsigned int)(end_fsb - offset_fsb),
> > -			XFS_DATA_FORK);
> > +	if (xfs_sb_version_hasrmapbt(&ip->i_mount->m_sb))
> > +		resblks = XFS_SWAP_RMAP_SPACE_RES(ip->i_mount,
> > +				(unsigned int)(end_fsb - offset_fsb),
> > +				XFS_DATA_FORK);
> > +	else
> > +		resblks = XFS_NEXTENTADD_SPACE_RES(ip->i_mount,
> > +				(unsigned int)(end_fsb - offset_fsb),
> > +				XFS_DATA_FORK);
> >  	error = xfs_trans_alloc(ip->i_mount, &M_RES(ip->i_mount)->tr_write,
> >  			resblks, 0, XFS_TRANS_RESERVE | XFS_TRANS_NOFS, &tp);
> >  	if (error)