Re: [PATCH] xfs: fix log reservation overflows when allocating large rt extents

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

On Fri, Dec 13, 2019 at 07:18:40AM -0500, Brian Foster wrote:
> On Wed, Dec 04, 2019 at 08:38:09AM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Omar Sandoval reported that a 4G fallocate on the realtime device causes
> > filesystem shutdowns due to a log reservation overflow that happens when
> > we log the rtbitmap updates.  Factor rtbitmap/rtsummary updates into the
> > the tr_write and tr_itruncate log reservation calculation.
> > 
> > "The following reproducer results in a transaction log overrun warning
> > for me:
> > 
> >     mkfs.xfs -f -r rtdev=/dev/vdc -d rtinherit=1 -m reflink=0 /dev/vdb
> >     mount -o rtdev=/dev/vdc /dev/vdb /mnt
> >     fallocate -l 4G /mnt/foo
> > 
> > Reported-by: Omar Sandoval <osandov@osandov.com>
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> 
> Looks reasonable enough given my limited knowledge on the rt bits. One
> question..
> 
> >  fs/xfs/libxfs/xfs_trans_resv.c |   96 ++++++++++++++++++++++++++++++++--------
> >  1 file changed, 77 insertions(+), 19 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_trans_resv.c b/fs/xfs/libxfs/xfs_trans_resv.c
> > index c55cd9a3dec9..824073a839ac 100644
> > --- a/fs/xfs/libxfs/xfs_trans_resv.c
> > +++ b/fs/xfs/libxfs/xfs_trans_resv.c
> > @@ -196,6 +196,24 @@ xfs_calc_inode_chunk_res(
> >  	return res;
> >  }
> >  
> > +/*
> > + * Per-extent log reservation for the btree changes involved in freeing or
> > + * allocating a realtime extent.  We have to be able to log as many rtbitmap
> > + * blocks as needed to mark inuse MAXEXTLEN blocks' worth of realtime extents,
> > + * as well as the realtime summary block.
> > + */
> > +unsigned int
> > +xfs_rtalloc_log_count(
> > +	struct xfs_mount	*mp,
> > +	unsigned int		num_ops)
> > +{
> > +	unsigned int		blksz = XFS_FSB_TO_B(mp, 1);
> > +	unsigned int		rtbmp_bytes;
> > +
> > +	rtbmp_bytes = (MAXEXTLEN / mp->m_sb.sb_rextsize) / NBBY;
> > +	return (howmany(rtbmp_bytes, blksz) + 1) * num_ops;
> > +}
> > +
> >  /*
> >   * Various log reservation values.
> >   *
> > @@ -218,13 +236,21 @@ xfs_calc_inode_chunk_res(
> >  
> >  /*
> >   * In a write transaction we can allocate a maximum of 2
> > - * extents.  This gives:
> > + * extents.  This gives (t1):
> >   *    the inode getting the new extents: inode size
> >   *    the inode's bmap btree: max depth * block size
> >   *    the agfs of the ags from which the extents are allocated: 2 * sector
> >   *    the superblock free block counter: sector size
> >   *    the allocation btrees: 2 exts * 2 trees * (2 * max depth - 1) * block size
> > - * And the bmap_finish transaction can free bmap blocks in a join:
> > + * Or, if we're writing to a realtime file (t2):
> > + *    the inode getting the new extents: inode size
> > + *    the inode's bmap btree: max depth * block size
> > + *    the agfs of the ags from which the extents are allocated: 2 * sector
> > + *    the superblock free block counter: sector size
> > + *    the realtime bitmap: ((MAXEXTLEN / rtextsize) / NBBY) bytes
> > + *    the realtime summary: 1 block
> > + *    the allocation btrees: 2 trees * (2 * max depth - 1) * block size
> 
> Why do we include the allocation btrees in the rt reservations? I
> thought that we'd either allocate (or free) out of one pool or the
> other. Do we operate on both sets of structures in the same transaction?

I read "allocation btrees: 2 exts * 2 trees..." for t1 to mean that we
need to be able to allocate one datadev extent (which could cause a full
bnobt/cntbt split) for the actual file data, and then the second extent is
to handle allocating a new bmbt block to the bmap btree.

Based on that, I concluded that we still need to reserve space for that
"second" extent to handle allocating a new bmbt block (on the datadev).

While pondering that, I wondered if even that's really true because what
happens if you suffer a full bmbt split, the free space is so fragmented
that each level of the bmbt split ends up allocating a new block from a
different part of the free space btrees and that in turn causes splits
in the free space btrees?

I think the answer is "no we're fine" because even if each new bmbt
block comes from a different bnobt record, a full bmbt split will never
hollow out more than half of one bnobt block's worth of free space
records.

--D

> Brian
> 
> > + * And the bmap_finish transaction can free bmap blocks in a join (t3):
> >   *    the agfs of the ags containing the blocks: 2 * sector size
> >   *    the agfls of the ags containing the blocks: 2 * sector size
> >   *    the super block free block counter: sector size
> > @@ -234,40 +260,72 @@ STATIC uint
> >  xfs_calc_write_reservation(
> >  	struct xfs_mount	*mp)
> >  {
> > -	return XFS_DQUOT_LOGRES(mp) +
> > -		max((xfs_calc_inode_res(mp, 1) +
> > +	unsigned int		t1, t2, t3;
> > +	unsigned int		blksz = XFS_FSB_TO_B(mp, 1);
> > +
> > +	t1 = xfs_calc_inode_res(mp, 1) +
> > +	     xfs_calc_buf_res(XFS_BM_MAXLEVELS(mp, XFS_DATA_FORK), blksz) +
> > +	     xfs_calc_buf_res(3, mp->m_sb.sb_sectsize) +
> > +	     xfs_calc_buf_res(xfs_allocfree_log_count(mp, 2), blksz);
> > +
> > +	if (xfs_sb_version_hasrealtime(&mp->m_sb)) {
> > +		t2 = xfs_calc_inode_res(mp, 1) +
> >  		     xfs_calc_buf_res(XFS_BM_MAXLEVELS(mp, XFS_DATA_FORK),
> > -				      XFS_FSB_TO_B(mp, 1)) +
> > +				      blksz) +
> >  		     xfs_calc_buf_res(3, mp->m_sb.sb_sectsize) +
> > -		     xfs_calc_buf_res(xfs_allocfree_log_count(mp, 2),
> > -				      XFS_FSB_TO_B(mp, 1))),
> > -		    (xfs_calc_buf_res(5, mp->m_sb.sb_sectsize) +
> > -		     xfs_calc_buf_res(xfs_allocfree_log_count(mp, 2),
> > -				      XFS_FSB_TO_B(mp, 1))));
> > +		     xfs_calc_buf_res(xfs_rtalloc_log_count(mp, 1), blksz) +
> > +		     xfs_calc_buf_res(xfs_allocfree_log_count(mp, 1), blksz);
> > +	} else {
> > +		t2 = 0;
> > +	}
> > +
> > +	t3 = xfs_calc_buf_res(5, mp->m_sb.sb_sectsize) +
> > +	     xfs_calc_buf_res(xfs_allocfree_log_count(mp, 2), blksz);
> > +
> > +	return XFS_DQUOT_LOGRES(mp) + max3(t1, t2, t3);
> >  }
> >  
> >  /*
> > - * In truncating a file we free up to two extents at once.  We can modify:
> > + * In truncating a file we free up to two extents at once.  We can modify (t1):
> >   *    the inode being truncated: inode size
> >   *    the inode's bmap btree: (max depth + 1) * block size
> > - * And the bmap_finish transaction can free the blocks and bmap blocks:
> > + * And the bmap_finish transaction can free the blocks and bmap blocks (t2):
> >   *    the agf for each of the ags: 4 * sector size
> >   *    the agfl for each of the ags: 4 * sector size
> >   *    the super block to reflect the freed blocks: sector size
> >   *    worst case split in allocation btrees per extent assuming 4 extents:
> >   *		4 exts * 2 trees * (2 * max depth - 1) * block size
> > + * Or, if it's a realtime file (t3):
> > + *    the agf for each of the ags: 2 * sector size
> > + *    the agfl for each of the ags: 2 * sector size
> > + *    the super block to reflect the freed blocks: sector size
> > + *    the realtime bitmap: 2 exts * ((MAXEXTLEN / rtextsize) / NBBY) bytes
> > + *    the realtime summary: 2 exts * 1 block
> > + *    worst case split in allocation btrees per extent assuming 2 extents:
> > + *		2 exts * 2 trees * (2 * max depth - 1) * block size
> >   */
> >  STATIC uint
> >  xfs_calc_itruncate_reservation(
> >  	struct xfs_mount	*mp)
> >  {
> > -	return XFS_DQUOT_LOGRES(mp) +
> > -		max((xfs_calc_inode_res(mp, 1) +
> > -		     xfs_calc_buf_res(XFS_BM_MAXLEVELS(mp, XFS_DATA_FORK) + 1,
> > -				      XFS_FSB_TO_B(mp, 1))),
> > -		    (xfs_calc_buf_res(9, mp->m_sb.sb_sectsize) +
> > -		     xfs_calc_buf_res(xfs_allocfree_log_count(mp, 4),
> > -				      XFS_FSB_TO_B(mp, 1))));
> > +	unsigned int		t1, t2, t3;
> > +	unsigned int		blksz = XFS_FSB_TO_B(mp, 1);
> > +
> > +	t1 = xfs_calc_inode_res(mp, 1) +
> > +	     xfs_calc_buf_res(XFS_BM_MAXLEVELS(mp, XFS_DATA_FORK) + 1, blksz);
> > +
> > +	t2 = xfs_calc_buf_res(9, mp->m_sb.sb_sectsize) +
> > +	     xfs_calc_buf_res(xfs_allocfree_log_count(mp, 4), blksz);
> > +
> > +	if (xfs_sb_version_hasrealtime(&mp->m_sb)) {
> > +		t3 = xfs_calc_buf_res(5, mp->m_sb.sb_sectsize) +
> > +		     xfs_calc_buf_res(xfs_rtalloc_log_count(mp, 2), blksz) +
> > +		     xfs_calc_buf_res(xfs_allocfree_log_count(mp, 2), blksz);
> > +	} else {
> > +		t3 = 0;
> > +	}
> > +
> > +	return XFS_DQUOT_LOGRES(mp) + max3(t1, t2, t3);
> >  }
> >  
> >  /*
> > 
>