Re: [PATCH V5] xfs: Avoid races with cnt_btree lastrec updates

[yangerkun <yangerkun@xxxxxxxxxx>]

在 2024/7/11 11:01, Darrick J. Wong 写道:
> On Wed, Jul 10, 2024 at 11:10:02AM +0800, yangerkun wrote:
> > Hi,
> >
> > 在 2024/7/1 14:02, Zizhi Wo 写道:
> > > 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 -current- 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 may 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. Move the cursor to the rightmost
> > >      firstly, and update the longest free extent based on the record.
> > >
> > > Note that we can not update the longest with xfs_alloc_get_rec() after
> > > find the longest record, as xfs_verify_agbno() may not pass because
> > > pag->block_count is updated on the outside. Therefore, use
> > > xfs_btree_get_rec() as a replacement.
> > >
> > > [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       | 114 ++++++++++++++++++++++++++++++++
> > >    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, 115 insertions(+), 130 deletions(-)
> > >
> > > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > > index 6c55a6e88eba..88fceb7ef946 100644
> > > --- a/fs/xfs/libxfs/xfs_alloc.c
> > > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > > @@ -465,6 +465,97 @@ 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 a record in this block we
> > > + * 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);
> > > +	return xfs_btree_ptr_is_null(cnt_cur, &ptr);
> > > +}
> > > +
> > > +/*
> > > + * Find the rightmost record of the cntbt, and return the longest free space
> > > + * recorded in it. Simply set both the block number and the length to their
> > > + * maximum values before searching.
> > > + */
> > > +static int
> > > +xfs_cntbt_longest(
> > > +	struct xfs_btree_cur	*cnt_cur,
> > > +	xfs_extlen_t		*longest)
> > > +{
> > > +	struct xfs_alloc_rec_incore irec;
> > > +	union xfs_btree_rec	    *rec;
> > > +	int			    stat = 0;
> > > +	int			    error;
> > > +
> > > +	memset(&cnt_cur->bc_rec, 0xFF, sizeof(cnt_cur->bc_rec));
> > > +	error = xfs_btree_lookup(cnt_cur, XFS_LOOKUP_LE, &stat);
> >
> > This seems a little hack now, can we use something like below to
> > help find the last record?
> >
> > error = xfs_btree_dup_cursor(cur, &tcur);
>
> Doesn't xfs_cntbt_longest get called on cnt_cur right before we delete
> the cursor?  In which case duplicating the cursor is unnecessary.
>
> > i = xfs_btree_lastrec(tcur, level);
>
> xfs_btree_lastrec moves the cursor to the last record in the current
> block, not the entire tree.  Is it always the case that the two are the
> same thing wherever xfs_cntbt_longest is called?

Yeah, thanks for point out this, I have not carefully check the detail
for this function, the function name make me think it will find the last
record in the tree...

> --D
>
> > Thanks,
> > Yang Erkun.
> >
> >
> > > +	if (error)
> > > +		return error;
> > > +	if (!stat) {
> > > +		/* totally empty tree */
> > > +		*longest = 0;
> > > +		return 0;
> > > +	}
> > > +
> > > +	error = xfs_btree_get_rec(cnt_cur, &rec, &stat);
> > > +	if (error)
> > > +		return error;
> > > +	if (XFS_IS_CORRUPT(cnt_cur->bc_mp, !stat)) {
> > > +		xfs_btree_mark_sick(cnt_cur);
> > > +		return -EFSCORRUPTED;
> > > +	}
> > > +
> > > +	xfs_alloc_btrec_to_irec(rec, &irec);
> > > +	*longest = irec.ar_blockcount;
> > > +	return 0;
> > > +}
> > > +
> > > +/*
> > > + * 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)
> > > +{
> > > +	struct xfs_perag	*pag = cnt_cur->bc_ag.pag;
> > > +	struct xfs_buf		*bp = cnt_cur->bc_ag.agbp;
> > > +	struct xfs_agf		*agf = bp->b_addr;
> > > +	xfs_extlen_t		longest = 0;
> > > +	int			error;
> > > +
> > > +	/* Lookup last rec in order to update AGF. */
> > > +	error = xfs_cntbt_longest(cnt_cur, &longest);
> > > +	if (error)
> > > +		return error;
> > > +
> > > +	pag->pagf_longest = longest;
> > > +	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 +580,7 @@ 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 +669,10 @@ 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 +750,10 @@ xfs_alloc_fixup_trees(
> > >    			return -EFSCORRUPTED;
> > >    		}
> > >    	}
> > > +
> > > +	if (fixup_longest)
> > > +		return xfs_alloc_fixup_longest(cnt_cur);
> > > +
> > >    	return 0;
> > >    }
> > > @@ -1956,6 +2056,7 @@ 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 +2320,13 @@ 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 +2335,8 @@ 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 +2344,12 @@ xfs_free_ag_extent(
> > >    		error = -EFSCORRUPTED;
> > >    		goto error0;
> > >    	}
> > > +	if (fixup_longest) {
> > > +		error = xfs_alloc_fixup_longest(cnt_cur);
> > > +		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..10b7ddc3b2b3 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 +216,7 @@ 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 */
> > > -
> > > -/*
> > > - * Reasons for the update_lastrec method to be called.
> > > - */
> > > -#define LASTREC_UPDATE	0
> > > -#define LASTREC_INSREC	1
> > > -#define LASTREC_DELREC	2
> > > +#define XFS_BTGEO_OVERLAPPING		(1U << 0) /* overlapping intervals */
> > >    union xfs_btree_irec {