xfs: Avoid races with cnt_btree lastrec updates

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@huawei.com
[2] https://lore.kernel.org/all/20240604071121.3981686-1-wozizhi@huawei.com

Reported by: Ye Bin <yebin10@huawei.com>

Signed-off-by: Zizhi Wo <wozizhi@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

1 files changed, 0 insertions, 51 deletions

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);
@@ -3618,15 +3585,6 @@ xfs_btree_insrec(
 	}
 
 	/*
-	 * 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.
 	 */
@@ -3984,15 +3942,6 @@ xfs_btree_delrec(
 	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
 	 * nothing left to do.