Re: [PATCH V3 09/12] xfs: Enable bulkstat ioctl to support 64-bit per-inode extent counters

On Tue, Sep 28, 2021 at 03:19:29PM +0530, Chandan Babu R wrote:

On 28 Sep 2021 at 04:36, Dave Chinner wrote:

quoted

On Thu, Sep 16, 2021 at 03:36:44PM +0530, Chandan Babu R wrote:

quoted

@@ -492,9 +494,16 @@ struct xfs_bulk_ireq {
  */
 #define XFS_BULK_IREQ_METADIR	(1 << 2)
 
-#define XFS_BULK_IREQ_FLAGS_ALL	(XFS_BULK_IREQ_AGNO | \
+#define XFS_BULK_IREQ_BULKSTAT	(1 << 3)
+
+#define XFS_BULK_IREQ_FLAGS_ALL	(XFS_BULK_IREQ_AGNO |	 \
 				 XFS_BULK_IREQ_SPECIAL | \
-				 XFS_BULK_IREQ_METADIR)
+				 XFS_BULK_IREQ_METADIR | \
+				 XFS_BULK_IREQ_BULKSTAT)

What's this XFS_BULK_IREQ_METADIR thing? I haven't noticed that when
scanning any recent proposed patch series....

XFS_BULK_IREQ_METADIR is from Darrick's tree. His "Kill XFS_BTREE_MAXLEVELS"
patch series is based on his other patchsets. His recent "xfs: support dynamic
btree cursor height" patch series rebases only the required patchset on top of
v5.15-rc1 kernel eliminating the others.

OK, so how much testing has this had on just a straight v5.15-rcX
kernel?

quoted

quoted

@@ -134,7 +136,26 @@ xfs_bulkstat_one_int(
 
 	buf->bs_xflags = xfs_ip2xflags(ip);
 	buf->bs_extsize_blks = ip->i_extsize;
-	buf->bs_extents = xfs_ifork_nextents(&ip->i_df);
+
+	nextents = xfs_ifork_nextents(&ip->i_df);
+	if (!(bc->breq->flags & XFS_IBULK_NREXT64)) {
+		xfs_extnum_t max_nextents = XFS_IFORK_EXTCNT_MAXS32;
+
+		if (unlikely(XFS_TEST_ERROR(false, mp,
+				XFS_ERRTAG_REDUCE_MAX_IEXTENTS)))
+			max_nextents = 10;
+
+		if (nextents > max_nextents) {
+			xfs_iunlock(ip, XFS_ILOCK_SHARED);
+			xfs_irele(ip);
+			error = -EINVAL;
+			goto out_advance;
+		}

So we return an EINVAL error if any extent overflows the 32 bit
counter? Why isn't this -EOVERFLOW?

Returning -EINVAL causes xfs_bulkstat_iwalk() to skip inodes whose extent
count is larger than that which can be fitted into a 32-bit field. Returning
-EOVERFLOW causes the bulkstat ioctl to stop reporting remaining inodes.

Ok, that's a bad behaviour we need to fix because it will cause
things like old versions of xfs_dump to miss inodes that
have overflowing extent counts. i.e. it will cause incomplete
backups, and the failure will likely be silent.

I asked about -EOVERFLOW because that's what stat() returns when an
inode attribute value doesn't fit in the stat_buf field (e.g. 64 bit
inode number on 32 bit kernel), and if we are overflowing the
bulkstat field then we really should be telling userspace that an
overflow occurred.

/me has a sudden realisation that the xfs_dump format may not
support large extents counts and goes looking...

Yeah, xfsdump doesn't support extent counts greater than 2^32. So
that means we really do need -EOVERFLOW errors here.  i.e, if we get
an extent count overflow with a !(bc->breq->flags &
XFS_IBULK_NREXT64) bulkstat walk, xfs_dump needs bulkstat to fill
out the inode with the overflow with all the fileds that aren't
overflowed, then error out with -EOVERFLOW.

Bulkstat itself should not silently skip the inode because it would
overflow a field in the struct xfs-bstat - the decision of what to
do with the overflow is something xfsdump needs to handle, not the
kernel.  Hence we need to return -EOVERFLOW here so that userspace
can decide what to do with an inode it can't handle...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com