xfs: only allocate memory for scrubbing attributes when we need it

In examining a flame graph of time spent running xfs_scrub on various
filesystems, I noticed that we spent nearly 7% of the total runtime on
allocating a zeroed 65k buffer for every SCRUB_TYPE_XATTR invocation.
We do this even if none of the attribute values were anywhere near 64k
in size, even if there were no attribute blocks to check space on, and
even if it just turns out there are no attributes at all.

Therefore, rearrange the xattr buffer setup code to support reallocating
with a bigger buffer and redistribute the callers of that function so
that we only allocate memory just prior to needing it, and only allocate
as much as we need.  If we can't get memory with the ILOCK held we'll
bail out with EDEADLOCK which will allocate the maximum memory.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/scrub/attr.c

@@ -20,13 +20,19 @@
 #include "scrub/dabtree.h"
 #include "scrub/attr.h"
 
-/* Allocate enough memory to hold an attr value and attr block bitmaps. */
+/*
+ * Allocate enough memory to hold an attr value and attr block bitmaps,
+ * reallocating the buffer if necessary.  Buffer contents are not preserved
+ * across a reallocation.
+ */
 int
 xchk_setup_xattr_buf(
 	struct xfs_scrub	*sc,
-	size_t			value_size)
+	size_t			value_size,
+	xfs_km_flags_t		flags)
 {
 	size_t			sz;
+	struct xchk_xattr_buf	*ab = sc->buf;
 
 	/*
 	 * We need enough space to read an xattr value from the file or enough
@@ -36,10 +42,23 @@ xchk_setup_xattr_buf(
 	sz = 3 * sizeof(long) * BITS_TO_LONGS(sc->mp->m_attr_geo->blksize);
 	sz = max_t(size_t, sz, value_size);
 
-	sc->buf = kmem_zalloc_large(sz, KM_SLEEP);
-	if (!sc->buf)
+	/*
+	 * If there's already a buffer, figure out if we need to reallocate it
+	 * to accommodate a larger size.
+	 */
+	if (ab) {
+		if (sz <= ab->sz)
+			return 0;
+		kmem_free(ab);
+		sc->buf = NULL;
+	}
+
+	ab = kmem_zalloc_large(sizeof(*ab) + sz, flags);
+	if (!ab)
 		return -ENOMEM;
 
+	ab->sz = sz;
+	sc->buf = ab;
 	return 0;
 }
 
@@ -51,9 +70,16 @@ xchk_setup_xattr(
 {
 	int			error;
 
-	error = xchk_setup_xattr_buf(sc, XATTR_SIZE_MAX);
-	if (error)
-		return error;
+	/*
+	 * We failed to get memory while checking attrs, so this time try to
+	 * get all the memory we're ever going to need.  Allocate the buffer
+	 * without the inode lock held, which means we can sleep.
+	 */
+	if (sc->flags & XCHK_TRY_HARDER) {
+		error = xchk_setup_xattr_buf(sc, XATTR_SIZE_MAX, KM_SLEEP);
+		if (error)
+			return error;
+	}
 
 	return xchk_setup_inode_contents(sc, ip, 0);
 }
@@ -104,6 +130,19 @@ xchk_xattr_listent(
 		return;
 	}
 
+	/*
+	 * Try to allocate enough memory to extrat the attr value.  If that
+	 * doesn't work, we overload the seen_enough variable to convey
+	 * the error message back to the main scrub function.
+	 */
+	error = xchk_setup_xattr_buf(sx->sc, valuelen, KM_MAYFAIL);
+	if (error == -ENOMEM)
+		error = -EDEADLOCK;
+	if (error) {
+		context->seen_enough = error;
+		return;
+	}
+
 	args.flags = ATTR_KERNOTIME;
 	if (flags & XFS_ATTR_ROOT)
 		args.flags |= ATTR_ROOT;
@@ -117,7 +156,7 @@ xchk_xattr_listent(
 	args.hashval = xfs_da_hashname(args.name, args.namelen);
 	args.trans = context->tp;
 	args.value = xchk_xattr_valuebuf(sx->sc);
-	args.valuelen = XATTR_SIZE_MAX;
+	args.valuelen = valuelen;
 
 	error = xfs_attr_get_ilocked(context->dp, &args);
 	if (error == -EEXIST)
@@ -270,16 +309,26 @@ xchk_xattr_block(
 	struct xfs_attr_leafblock	*leaf = bp->b_addr;
 	struct xfs_attr_leaf_entry	*ent;
 	struct xfs_attr_leaf_entry	*entries;
-	unsigned long			*usedmap = xchk_xattr_usedmap(ds->sc);
+	unsigned long			*usedmap;
 	char				*buf_end;
 	size_t				off;
 	__u32				last_hashval = 0;
 	unsigned int			usedbytes = 0;
 	unsigned int			hdrsize;
 	int				i;
+	int				error;
 
 	if (*last_checked == blk->blkno)
 		return 0;
+
+	/* Allocate memory for block usage checking. */
+	error = xchk_setup_xattr_buf(ds->sc, 0, KM_MAYFAIL);
+	if (error == -ENOMEM)
+		return -EDEADLOCK;
+	if (error)
+		return error;
+	usedmap = xchk_xattr_usedmap(ds->sc);
+
 	*last_checked = blk->blkno;
 	bitmap_zero(usedmap, mp->m_attr_geo->blksize);
 

fs/xfs/scrub/attr.h

@@ -10,6 +10,9 @@
  * Temporary storage for online scrub and repair of extended attributes.
  */
 struct xchk_xattr_buf {
+	/* Size of @buf, in bytes. */
+	size_t			sz;
+
 	/*
 	 * Memory buffer -- either used for extracting attr values while
 	 * walking the attributes; or for computing attr block bitmaps when
@@ -62,6 +65,7 @@ xchk_xattr_dstmap(
 			BITS_TO_LONGS(sc->mp->m_attr_geo->blksize);
 }
 
-int xchk_setup_xattr_buf(struct xfs_scrub *sc, size_t value_size);
+int xchk_setup_xattr_buf(struct xfs_scrub *sc, size_t value_size,
+		xfs_km_flags_t flags);
 
 #endif	/* __XFS_SCRUB_ATTR_H__ */