Re: [PATCH] xfs: Avoid races with cnt_btree lastrec updates

[Zizhi Wo <wozizhi@xxxxxxxxxx>]

在 2024/6/19 8:22, Darrick J. Wong 写道:
> On Tue, Jun 18, 2024 at 09:32:08PM +0800, Zizhi Wo wrote:
> > A concurrent file creation and little writing could unexpectedly return
> > -ENOSPC error since there is a race window that the allocator could get
> > the wrong agf->agf_longest.
> >
> > Write file process steps:
> > 1) Find the entry that best meets the conditions, then calculate the start
> >     address and length of the remaining part of the entry after allocation.
> > 2) Delete this entry and update the agf->agf_longest.
> > 3) Insert the remaining unused parts of this entry based on the
> >     calculations in 1), and update the agf->agf_longest again if necessary.
> >
> > Create file process steps:
> > 1) Check whether there are free inodes in the inode chunk.
> > 2) If there is no free inode, check whether there has space for creating
> >     inode chunks, perform the no-lock judgment first.
> > 3) If the judgment succeeds, the judgment is performed again with agf lock
> >     held. Otherwire, an error is returned directly.
> >
> > If the write process is in step 2) but not go to 3) yet, the create file
> > process goes to 2) at this time, it will be mistaken for no space,
> > resulting in the file system still has space but the file creation fails.
> >
> > We have sent two different commits to the community in order to fix this
> > problem[1][2]. Unfortunately, both solutions have flaws. In [2], I
> > discussed with Dave and Darrick, realized that a better solution to this
> > problem requires the "last cnt record tracking" to be ripped out of the
> > generic btree code. And surprisingly, Dave directly provided his fix code.
> > This patch includes appropriate modifications based on his tmp-code to
> > address this issue.
> >
> > The entire fix can be roughly divided into two parts:
> > 1) Delete the code related to lastrec-update in the generic btree code.
> > 2) Place the process of updating longest freespace with cntbt separately
> >     to the end of the cntbt modifications. And only these two scenarios
> >     need to be considered:
> >     2.1) In the deletion scenario, directly update the longest to the
> >          rightmost record of the cntbt.
> >     2.2) In the insertion scenario, determine whether the cntbt has the
> >          record that larger than the previous longest.
> >
> > [1] https://lore.kernel.org/all/20240419061848.1032366-2-yebin10@xxxxxxxxxx
> > [2] https://lore.kernel.org/all/20240604071121.3981686-1-wozizhi@xxxxxxxxxx
> >
> > Reported by: Ye Bin <yebin10@xxxxxxxxxx>
> > Signed-off-by: Zizhi Wo <wozizhi@xxxxxxxxxx>
> > ---
> >   fs/xfs/libxfs/xfs_alloc.c       | 116 ++++++++++++++++++++++++++++++++
> >   fs/xfs/libxfs/xfs_alloc_btree.c |  64 ------------------
> >   fs/xfs/libxfs/xfs_btree.c       |  51 --------------
> >   fs/xfs/libxfs/xfs_btree.h       |  16 ++---
> >   4 files changed, 120 insertions(+), 127 deletions(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > index 6c55a6e88eba..74e40f75a278 100644
> > --- a/fs/xfs/libxfs/xfs_alloc.c
> > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > @@ -465,6 +465,99 @@ xfs_alloc_fix_len(
> >   	args->len = rlen;
> >   }
> >   
> > +/*
> > + * Determine if the cursor points to the block that contains the right-most
> > + * block of records in the by-count btree. This block contains the largest
> > + * contiguous free extent in the AG, so if we modify ia record in this block we
>
>                                                          a record
>
> > + * need to call xfs_alloc_fixup_longest() once the modifications are done to
> > + * ensure the agf->agf_longest field is kept up to date with the longest free
> > + * extent tracked by the by-count btree.
> > + */
> > +static bool
> > +xfs_alloc_cursor_at_lastrec(
> > +	struct xfs_btree_cur	*cnt_cur)
> > +{
> > +	struct xfs_btree_block	*block;
> > +	union xfs_btree_ptr	ptr;
> > +	struct xfs_buf		*bp;
> > +
> > +	block = xfs_btree_get_block(cnt_cur, 0, &bp);
> > +
> > +	xfs_btree_get_sibling(cnt_cur, block, &ptr, XFS_BB_RIGHTSIB);
> > +	if (!xfs_btree_ptr_is_null(cnt_cur, &ptr))
> > +		return false;
> > +	return true;
> > +}
> > +
> > +/*
> > + * Update the longest contiguous free extent in the AG from the by-count cursor
> > + * that is passed to us. This should be done at the end of any allocation or
> > + * freeing operation that touches the longest extent in the btree.
> > + *
> > + * Needing to update the longest extent can be determined by calling
> > + * xfs_alloc_cursor_at_lastrec() after the cursor is positioned for record
> > + * modification but before the modification begins.
> > + */
> > +static int
> > +xfs_alloc_fixup_longest(
> > +	struct xfs_btree_cur	*cnt_cur,
> > +	int			reason)
> > +{
> > +	struct xfs_perag	*pag = cnt_cur->bc_ag.pag;
> > +	struct xfs_agf		*agf;
> > +	struct xfs_buf		*bp;
> > +	struct xfs_btree_block	*block;
> > +	int			error;
> > +	int			i;
> > +	int			numrecs;
> > +
> > +	/*
> > +	 * Lookup last rec and update AGF.
> > +	 *
> > +	 * In case of LASTREC_DELREC, after called xfs_alloc_lookup_ge(), the
> > +	 * ptr is in the rightmost edge, and we need to update the last record
> > +	 * of this block as the longest free extent.
> > +	 *
> > +	 * In case of LASTREC_INSREC, because only one new record is inserted
> > +	 * each time, only need to check whether the cntbt has a record that
> > +	 * larger than the previous longest. Note that we can't update the
> > +	 * longest with xfs_alloc_get_rec() as the xfs_verify_agbno() may not
> > +	 * pass because pag->block_count is updated on the outside.
> > +	 */
> > +	error = xfs_alloc_lookup_ge(cnt_cur, 0, pag->pagf_longest + 1, &i);
> > +	if (error)
> > +		return error;
> > +
> > +	if (i == 1 || reason == LASTREC_DELREC) {
> > +		if (XFS_IS_CORRUPT(pag->pag_mount,
> > +				   i == 1 && reason == LASTREC_DELREC)) {
> > +			xfs_btree_mark_sick(cnt_cur);
> > +			return -EFSCORRUPTED;
> > +		}
> > +
> > +		block = xfs_btree_get_block(cnt_cur, 0, &bp);
> > +		numrecs = xfs_btree_get_numrecs(block);
> > +
> > +		if (numrecs) {
> > +			xfs_alloc_rec_t *rrp;
> > +
> > +			rrp = XFS_ALLOC_REC_ADDR(cnt_cur->bc_mp, block,
> > +						 numrecs);
> > +			pag->pagf_longest = be32_to_cpu(rrp->ar_blockcount);
> > +		} else {
> > +			/* empty tree */
> > +			pag->pagf_longest = 0;
> > +		}
> > +	}
>
> Hum.  Would it work if we did:
>
> 	xfs_extlen_t	len;
>
> 	xfs_alloc_lookup_le(cnt_cur, 0, mp->m_sb.sb_agsize, &i);
>
> 	if (i)
> 		xfs_alloc_get_rec(cnt_cur, ..., &len, &i);
> 	if (!i)
> 		len = 0;
>
> 	pag->pagf_longest = len;
>
> This performs a LE lookup on the longest possible free extent (aka the
> AG size).  If we get pointed at a record, that's the longest free extent
> and we can set pag->pagf_longest to that.  If we get no record, then
> there's zero space and we can zero it.

I checked it out, and it doesn't seem to work that way, for two reasons:
1) In the insertion scenario, if the longest extent is continuous with
the extent to be inserted, the system deletes the longest extent first,
and then inserts a more longer extent. So in the last update, we should
not look at le, but ge.
2) For the deletion scenario, because the start block is 0, the extent
value found is not the next longest extent value, but probably a very
small extent value (for example, its start block is very small), which
will also cause us to update incorrectly.

So in summary, I think it is appropriate to use ge here. On the other
hand, xfs_alloc_get_rec() introduces problems. As I wrote in the
comment, an internal call to xfs_verify_agbno() may fail because the
longest extent added, it could be greater than pag->block_count as
pag->block_count is later than the longest update.


> Then I think you don't need the @reason argument either.

Mm-hm, the reasons for this will be removed from the comment and stated
next time in the commit message.

Thanks
Zizhi Wo

> --D

> > +
> > +	bp = cnt_cur->bc_ag.agbp;
> > +	agf = bp->b_addr;
> > +	agf->agf_longest = cpu_to_be32(pag->pagf_longest);
> > +	xfs_alloc_log_agf(cnt_cur->bc_tp, bp, XFS_AGF_LONGEST);
> > +
> > +	return 0;
> > +}
> > +
> >   /*
> >    * Update the two btrees, logically removing from freespace the extent
> >    * starting at rbno, rlen blocks.  The extent is contained within the
> > @@ -489,6 +582,7 @@ xfs_alloc_fixup_trees(
> >   	xfs_extlen_t	nflen1=0;	/* first new free length */
> >   	xfs_extlen_t	nflen2=0;	/* second new free length */
> >   	struct xfs_mount *mp;
> > +	bool		fixup_longest = false;
> >   
> >   	mp = cnt_cur->bc_mp;
> >   
> > @@ -577,6 +671,10 @@ xfs_alloc_fixup_trees(
> >   		nfbno2 = rbno + rlen;
> >   		nflen2 = (fbno + flen) - nfbno2;
> >   	}
> > +
> > +	if (xfs_alloc_cursor_at_lastrec(cnt_cur))
> > +		fixup_longest = true;
> > +
> >   	/*
> >   	 * Delete the entry from the by-size btree.
> >   	 */
> > @@ -654,6 +752,10 @@ xfs_alloc_fixup_trees(
> >   			return -EFSCORRUPTED;
> >   		}
> >   	}
> > +
> > +	if (fixup_longest)
> > +		return xfs_alloc_fixup_longest(cnt_cur, LASTREC_DELREC);
> > +
> >   	return 0;
> >   }
> >   
> > @@ -1956,6 +2058,7 @@ xfs_free_ag_extent(
> >   	int				i;
> >   	int				error;
> >   	struct xfs_perag		*pag = agbp->b_pag;
> > +	bool				fixup_longest = false;
> >   
> >   	bno_cur = cnt_cur = NULL;
> >   	mp = tp->t_mountp;
> > @@ -2219,8 +2322,13 @@ xfs_free_ag_extent(
> >   	}
> >   	xfs_btree_del_cursor(bno_cur, XFS_BTREE_NOERROR);
> >   	bno_cur = NULL;
> > +
> >   	/*
> >   	 * In all cases we need to insert the new freespace in the by-size tree.
> > +	 *
> > +	 * If this new freespace is being inserted in the block that contains
> > +	 * the largest free space in the btree, make sure we also fix up the
> > +	 * agf->agf-longest tracker field.
> >   	 */
> >   	if ((error = xfs_alloc_lookup_eq(cnt_cur, nbno, nlen, &i)))
> >   		goto error0;
> > @@ -2229,6 +2337,8 @@ xfs_free_ag_extent(
> >   		error = -EFSCORRUPTED;
> >   		goto error0;
> >   	}
> > +	if (xfs_alloc_cursor_at_lastrec(cnt_cur))
> > +		fixup_longest = true;
> >   	if ((error = xfs_btree_insert(cnt_cur, &i)))
> >   		goto error0;
> >   	if (XFS_IS_CORRUPT(mp, i != 1)) {
> > @@ -2236,6 +2346,12 @@ xfs_free_ag_extent(
> >   		error = -EFSCORRUPTED;
> >   		goto error0;
> >   	}
> > +	if (fixup_longest) {
> > +		error = xfs_alloc_fixup_longest(cnt_cur, LASTREC_INSREC);
> > +		if (error)
> > +			goto error0;
> > +	}
> > +
> >   	xfs_btree_del_cursor(cnt_cur, XFS_BTREE_NOERROR);
> >   	cnt_cur = NULL;
> >   
> > diff --git a/fs/xfs/libxfs/xfs_alloc_btree.c b/fs/xfs/libxfs/xfs_alloc_btree.c
> > index 6ef5ddd89600..585e98e87ef9 100644
> > --- a/fs/xfs/libxfs/xfs_alloc_btree.c
> > +++ b/fs/xfs/libxfs/xfs_alloc_btree.c
> > @@ -115,67 +115,6 @@ xfs_allocbt_free_block(
> >   	return 0;
> >   }
> >   
> > -/*
> > - * Update the longest extent in the AGF
> > - */
> > -STATIC void
> > -xfs_allocbt_update_lastrec(
> > -	struct xfs_btree_cur		*cur,
> > -	const struct xfs_btree_block	*block,
> > -	const union xfs_btree_rec	*rec,
> > -	int				ptr,
> > -	int				reason)
> > -{
> > -	struct xfs_agf		*agf = cur->bc_ag.agbp->b_addr;
> > -	struct xfs_perag	*pag;
> > -	__be32			len;
> > -	int			numrecs;
> > -
> > -	ASSERT(!xfs_btree_is_bno(cur->bc_ops));
> > -
> > -	switch (reason) {
> > -	case LASTREC_UPDATE:
> > -		/*
> > -		 * If this is the last leaf block and it's the last record,
> > -		 * then update the size of the longest extent in the AG.
> > -		 */
> > -		if (ptr != xfs_btree_get_numrecs(block))
> > -			return;
> > -		len = rec->alloc.ar_blockcount;
> > -		break;
> > -	case LASTREC_INSREC:
> > -		if (be32_to_cpu(rec->alloc.ar_blockcount) <=
> > -		    be32_to_cpu(agf->agf_longest))
> > -			return;
> > -		len = rec->alloc.ar_blockcount;
> > -		break;
> > -	case LASTREC_DELREC:
> > -		numrecs = xfs_btree_get_numrecs(block);
> > -		if (ptr <= numrecs)
> > -			return;
> > -		ASSERT(ptr == numrecs + 1);
> > -
> > -		if (numrecs) {
> > -			xfs_alloc_rec_t *rrp;
> > -
> > -			rrp = XFS_ALLOC_REC_ADDR(cur->bc_mp, block, numrecs);
> > -			len = rrp->ar_blockcount;
> > -		} else {
> > -			len = 0;
> > -		}
> > -
> > -		break;
> > -	default:
> > -		ASSERT(0);
> > -		return;
> > -	}
> > -
> > -	agf->agf_longest = len;
> > -	pag = cur->bc_ag.agbp->b_pag;
> > -	pag->pagf_longest = be32_to_cpu(len);
> > -	xfs_alloc_log_agf(cur->bc_tp, cur->bc_ag.agbp, XFS_AGF_LONGEST);
> > -}
> > -
> >   STATIC int
> >   xfs_allocbt_get_minrecs(
> >   	struct xfs_btree_cur	*cur,
> > @@ -493,7 +432,6 @@ const struct xfs_btree_ops xfs_bnobt_ops = {
> >   	.set_root		= xfs_allocbt_set_root,
> >   	.alloc_block		= xfs_allocbt_alloc_block,
> >   	.free_block		= xfs_allocbt_free_block,
> > -	.update_lastrec		= xfs_allocbt_update_lastrec,
> >   	.get_minrecs		= xfs_allocbt_get_minrecs,
> >   	.get_maxrecs		= xfs_allocbt_get_maxrecs,
> >   	.init_key_from_rec	= xfs_allocbt_init_key_from_rec,
> > @@ -511,7 +449,6 @@ const struct xfs_btree_ops xfs_bnobt_ops = {
> >   const struct xfs_btree_ops xfs_cntbt_ops = {
> >   	.name			= "cnt",
> >   	.type			= XFS_BTREE_TYPE_AG,
> > -	.geom_flags		= XFS_BTGEO_LASTREC_UPDATE,
> >   
> >   	.rec_len		= sizeof(xfs_alloc_rec_t),
> >   	.key_len		= sizeof(xfs_alloc_key_t),
> > @@ -525,7 +462,6 @@ const struct xfs_btree_ops xfs_cntbt_ops = {
> >   	.set_root		= xfs_allocbt_set_root,
> >   	.alloc_block		= xfs_allocbt_alloc_block,
> >   	.free_block		= xfs_allocbt_free_block,
> > -	.update_lastrec		= xfs_allocbt_update_lastrec,
> >   	.get_minrecs		= xfs_allocbt_get_minrecs,
> >   	.get_maxrecs		= xfs_allocbt_get_maxrecs,
> >   	.init_key_from_rec	= xfs_allocbt_init_key_from_rec,
> > diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> > index d29547572a68..a5c4af148853 100644
> > --- a/fs/xfs/libxfs/xfs_btree.c
> > +++ b/fs/xfs/libxfs/xfs_btree.c
> > @@ -1331,30 +1331,6 @@ xfs_btree_init_block_cur(
> >   			xfs_btree_owner(cur));
> >   }
> >   
> > -/*
> > - * Return true if ptr is the last record in the btree and
> > - * we need to track updates to this record.  The decision
> > - * will be further refined in the update_lastrec method.
> > - */
> > -STATIC int
> > -xfs_btree_is_lastrec(
> > -	struct xfs_btree_cur	*cur,
> > -	struct xfs_btree_block	*block,
> > -	int			level)
> > -{
> > -	union xfs_btree_ptr	ptr;
> > -
> > -	if (level > 0)
> > -		return 0;
> > -	if (!(cur->bc_ops->geom_flags & XFS_BTGEO_LASTREC_UPDATE))
> > -		return 0;
> > -
> > -	xfs_btree_get_sibling(cur, block, &ptr, XFS_BB_RIGHTSIB);
> > -	if (!xfs_btree_ptr_is_null(cur, &ptr))
> > -		return 0;
> > -	return 1;
> > -}
> > -
> >   STATIC void
> >   xfs_btree_buf_to_ptr(
> >   	struct xfs_btree_cur	*cur,
> > @@ -2420,15 +2396,6 @@ xfs_btree_update(
> >   	xfs_btree_copy_recs(cur, rp, rec, 1);
> >   	xfs_btree_log_recs(cur, bp, ptr, ptr);
> >   
> > -	/*
> > -	 * If we are tracking the last record in the tree and
> > -	 * we are at the far right edge of the tree, update it.
> > -	 */
> > -	if (xfs_btree_is_lastrec(cur, block, 0)) {
> > -		cur->bc_ops->update_lastrec(cur, block, rec,
> > -					    ptr, LASTREC_UPDATE);
> > -	}
> > -
> >   	/* Pass new key value up to our parent. */
> >   	if (xfs_btree_needs_key_update(cur, ptr)) {
> >   		error = xfs_btree_update_keys(cur, 0);
> > @@ -3617,15 +3584,6 @@ xfs_btree_insrec(
> >   			goto error0;
> >   	}
> >   
> > -	/*
> > -	 * If we are tracking the last record in the tree and
> > -	 * we are at the far right edge of the tree, update it.
> > -	 */
> > -	if (xfs_btree_is_lastrec(cur, block, level)) {
> > -		cur->bc_ops->update_lastrec(cur, block, rec,
> > -					    ptr, LASTREC_INSREC);
> > -	}
> > -
> >   	/*
> >   	 * Return the new block number, if any.
> >   	 * If there is one, give back a record value and a cursor too.
> > @@ -3983,15 +3941,6 @@ xfs_btree_delrec(
> >   	xfs_btree_set_numrecs(block, --numrecs);
> >   	xfs_btree_log_block(cur, bp, XFS_BB_NUMRECS);
> >   
> > -	/*
> > -	 * If we are tracking the last record in the tree and
> > -	 * we are at the far right edge of the tree, update it.
> > -	 */
> > -	if (xfs_btree_is_lastrec(cur, block, level)) {
> > -		cur->bc_ops->update_lastrec(cur, block, NULL,
> > -					    ptr, LASTREC_DELREC);
> > -	}
> > -
> >   	/*
> >   	 * We're at the root level.  First, shrink the root block in-memory.
> >   	 * Try to get rid of the next level down.  If we can't then there's
> > diff --git a/fs/xfs/libxfs/xfs_btree.h b/fs/xfs/libxfs/xfs_btree.h
> > index f93374278aa1..670470874630 100644
> > --- a/fs/xfs/libxfs/xfs_btree.h
> > +++ b/fs/xfs/libxfs/xfs_btree.h
> > @@ -154,12 +154,6 @@ struct xfs_btree_ops {
> >   			       int *stat);
> >   	int	(*free_block)(struct xfs_btree_cur *cur, struct xfs_buf *bp);
> >   
> > -	/* update last record information */
> > -	void	(*update_lastrec)(struct xfs_btree_cur *cur,
> > -				  const struct xfs_btree_block *block,
> > -				  const union xfs_btree_rec *rec,
> > -				  int ptr, int reason);
> > -
> >   	/* records in block/level */
> >   	int	(*get_minrecs)(struct xfs_btree_cur *cur, int level);
> >   	int	(*get_maxrecs)(struct xfs_btree_cur *cur, int level);
> > @@ -222,15 +216,13 @@ struct xfs_btree_ops {
> >   };
> >   
> >   /* btree geometry flags */
> > -#define XFS_BTGEO_LASTREC_UPDATE	(1U << 0) /* track last rec externally */
> > -#define XFS_BTGEO_OVERLAPPING		(1U << 1) /* overlapping intervals */
> > +#define XFS_BTGEO_OVERLAPPING		(1U << 0) /* overlapping intervals */
> >   
> >   /*
> > - * Reasons for the update_lastrec method to be called.
> > + * Reasons for the xfs_alloc_fixup_longest() to be called.
> >    */
> > -#define LASTREC_UPDATE	0
> > -#define LASTREC_INSREC	1
> > -#define LASTREC_DELREC	2
> > +#define LASTREC_INSREC	0
> > +#define LASTREC_DELREC	1
> >   
> >   
> >   union xfs_btree_irec {
> > --
> > 2.39.2