xfs: allocate xattr buffer on demand

+/*
+ * Retrieve an extended attribute and its value. Must have ilock.
+ * Returns 0 on successful retrieval, otherwise an error.
+ */
int
xfs_attr_get_ilocked(

STATIC int
xfs_attr_node_get(xfs_da_args_t *args)

fs/xfs/libxfs/xfs_attr.c +10 -13

@@ -1294,24 +1306,27 @@ xfs_attr_node_get(xfs_da_args_t *args)
 	error = xfs_da3_node_lookup_int(state, &retval);
 	if (error) {
 		retval = error;
-	} else if (retval == -EEXIST) {
-		blk = &state->path.blk[ state->path.active-1 ];
-		ASSERT(blk->bp != NULL);
-		ASSERT(blk->magic == XFS_ATTR_LEAF_MAGIC);
-
-		/*
-		 * Get the value, local or "remote"
-		 */
-		retval = xfs_attr3_leaf_getvalue(blk->bp, args);
-		if (!retval && (args->rmtblkno > 0)
-		    && !(args->flags & ATTR_KERNOVAL)) {
-			retval = xfs_attr_rmtval_get(args);
-		}
+		goto out_release;
 	}
+	if (retval != -EEXIST)
+		goto out_release;
+
+	/*
+	 * Get the value, local or "remote"
+	 */
+	blk = &state->path.blk[state->path.active - 1];
+	retval = xfs_attr3_leaf_getvalue(blk->bp, args);
+	if (retval)

fs/xfs/libxfs/xfs_attr_leaf.c +1 -1

 int
 xfs_attr_shortform_getvalue(xfs_da_args_t *args)
 {

@@ -743,7 +746,7 @@ xfs_attr_shortform_getvalue(xfs_da_args_t *args)
 			continue;
 		if (args->flags & ATTR_KERNOVAL) {
 			args->valuelen = sfe->valuelen;
-			return -EEXIST;
+			return 0;
 		}
 		if (args->valuelen < sfe->valuelen) {
 			args->valuelen = sfe->valuelen;

From: Dave Chinner <[email protected]>

Signed-off-by Dave Chinner <[email protected]>

---
 fs/xfs/libxfs/xfs_attr_leaf.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

fs/xfs/libxfs/xfs_attr_leaf.c +17 -16

index 7a9440b7ab00..c7378bc62d2b 100644
@@ -2391,24 +2391,25 @@ xfs_attr3_leaf_getvalue(
 		}
 		args->valuelen = valuelen;
 		memcpy(args->value, &name_loc->nameval[args->namelen], valuelen);
-	} else {
-		name_rmt = xfs_attr3_leaf_name_remote(leaf, args->index);
-		ASSERT(name_rmt->namelen == args->namelen);
-		ASSERT(memcmp(args->name, name_rmt->name, args->namelen) == 0);
-		args->rmtvaluelen = be32_to_cpu(name_rmt->valuelen);
-		args->rmtblkno = be32_to_cpu(name_rmt->valueblk);
-		args->rmtblkcnt = xfs_attr3_rmt_blocks(args->dp->i_mount,
-						       args->rmtvaluelen);
-		if (args->flags & ATTR_KERNOVAL) {
-			args->valuelen = args->rmtvaluelen;
-			return 0;
-		}
-		if (args->valuelen < args->rmtvaluelen) {
-			args->valuelen = args->rmtvaluelen;
-			return -ERANGE;
-		}
+		return 0;
+	}
+
+	name_rmt = xfs_attr3_leaf_name_remote(leaf, args->index);
+	ASSERT(name_rmt->namelen == args->namelen);
+	ASSERT(memcmp(args->name, name_rmt->name, args->namelen) == 0);
+	args->rmtvaluelen = be32_to_cpu(name_rmt->valuelen);
+	args->rmtblkno = be32_to_cpu(name_rmt->valueblk);
+	args->rmtblkcnt = xfs_attr3_rmt_blocks(args->dp->i_mount,
+					       args->rmtvaluelen);
+	if (args->flags & ATTR_KERNOVAL) {
+		args->valuelen = args->rmtvaluelen;
+		return 0;
+	}
+	if (args->valuelen < args->rmtvaluelen) {
 		args->valuelen = args->rmtvaluelen;
+		return -ERANGE;
 	}
+	args->valuelen = args->rmtvaluelen;
 	return 0;
 }
 

Signature · 1 line

2.23.0.rc1

From: Dave Chinner <[email protected]>

Signed-off-by Dave Chinner <[email protected]>

From: Dave Chinner <[email protected]>

Because we repeat exactly the same code to get the remote attribute value after both calls to xfs_attr3_leaf_getvalue() if it's a remote attr. Just do it in xfs_attr3_leaf_getvalue() so the callers don't have to care about it.

Signed-off-by Dave Chinner <[email protected]>

---
 fs/xfs/libxfs/xfs_attr.c      | 16 +---------------
 fs/xfs/libxfs/xfs_attr_leaf.c |  2 +-
 2 files changed, 2 insertions(+), 16 deletions(-)

fs/xfs/libxfs/xfs_attr.c +1 -15

index 32879ab11290..4773eef9d3de 100644
@@ -794,15 +794,7 @@ xfs_attr_leaf_get(xfs_da_args_t *args)
 	}
 	error = xfs_attr3_leaf_getvalue(bp, args);
 	xfs_trans_brelse(args->trans, bp);
-	if (error)
-		return error;
-
-	/* check if we have to retrieve a remote attribute to get the value */
-	if (args->flags & ATTR_KERNOVAL)
-		return 0;
-	if (!args->rmtblkno)
-		return 0;
-	return xfs_attr_rmtval_get(args);
+	return error;
 }
 
 /*========================================================================

@@ -1316,12 +1308,6 @@ xfs_attr_node_get(xfs_da_args_t *args)
 	 */
 	blk = &state->path.blk[state->path.active - 1];
 	retval = xfs_attr3_leaf_getvalue(blk->bp, args);
-	if (retval)
-		goto out_release;
-	if (args->flags & ATTR_KERNOVAL)
-		goto out_release;
-	if (args->rmtblkno > 0)
-		retval = xfs_attr_rmtval_get(args);
 
 	/*
 	 * If not in a transaction, we have to release all the buffers.

fs/xfs/libxfs/xfs_attr_leaf.c +1 -1

index c7378bc62d2b..8085c4f0e5a0 100644
@@ -2410,7 +2410,7 @@ xfs_attr3_leaf_getvalue(
 		return -ERANGE;
 	}
 	args->valuelen = args->rmtvaluelen;
-	return 0;
+	return xfs_attr_rmtval_get(args);
 }
 
 /*========================================================================

Signature · 1 line

2.23.0.rc1

From: Dave Chinner <[email protected]>

Because we repeat exactly the same code to get the remote attribute value after both calls to xfs_attr3_leaf_getvalue() if it's a remote attr. Just do it in xfs_attr3_leaf_getvalue() so the callers don't have to care about it.

Signed-off-by Dave Chinner <[email protected]>

From: Dave Chinner <[email protected]>

The same code is used to copy do the attribute copying in three different places. Consolidate them into a single function in preparation from on-demand buffer allocation.

Signed-off-by Dave Chinner <[email protected]>

---
 fs/xfs/libxfs/xfs_attr_leaf.c | 88 +++++++++++++++++++----------------
 1 file changed, 49 insertions(+), 39 deletions(-)

fs/xfs/libxfs/xfs_attr_leaf.c +49 -39

index 8085c4f0e5a0..f6a595e76343 100644
@@ -393,6 +393,44 @@ xfs_attr_namesp_match(int arg_flags, int ondisk_flags)
 	return XFS_ATTR_NSP_ONDISK(ondisk_flags) == XFS_ATTR_NSP_ARGS_TO_ONDISK(arg_flags);
 }
 
+static int
+xfs_attr_copy_value(
+	struct xfs_da_args	*args,
+	unsigned char		*value,
+	int			valuelen)
+{
+	/*
+	 * No copy if all we have to do is get the length
+	 */
+	if (args->flags & ATTR_KERNOVAL) {
+		args->valuelen = valuelen;
+		return 0;
+	}
+
+	/*
+	 * No copy if the length of the existing buffer is too small
+	 */
+	if (args->valuelen < valuelen) {
+		args->valuelen = valuelen;
+		return -ERANGE;
+	}
+	args->valuelen = valuelen;
+
+	/* remote block xattr requires IO for copy-in */
+	if (args->rmtblkno)
+		return xfs_attr_rmtval_get(args);
+
+	/*
+	 * This is to prevent a GCC warning because the remote xattr case
+	 * doesn't have a value to pass in. In that case, we never reach here,
+	 * but GCC can't work that out and so throws a "passing NULL to
+	 * memcpy" warning.
+	 */
+	if (!value)
+		return -EINVAL;
+	memcpy(args->value, value, valuelen);
+	return 0;
+}
 
 /*========================================================================
  * External routines when attribute fork size < XFS_LITINO(mp).

@@ -727,11 +765,12 @@ xfs_attr_shortform_lookup(xfs_da_args_t *args)
  * exist or we can't retrieve the value.
  */
 int
-xfs_attr_shortform_getvalue(xfs_da_args_t *args)
+xfs_attr_shortform_getvalue(
+	struct xfs_da_args	*args)
 {
-	xfs_attr_shortform_t *sf;
-	xfs_attr_sf_entry_t *sfe;
-	int i;
+	struct xfs_attr_shortform *sf;
+	struct xfs_attr_sf_entry *sfe;
+	int			i;
 
 	ASSERT(args->dp->i_afp->if_flags == XFS_IFINLINE);
 	sf = (xfs_attr_shortform_t *)args->dp->i_afp->if_u1.if_data;

@@ -744,18 +783,8 @@ xfs_attr_shortform_getvalue(xfs_da_args_t *args)
 			continue;
 		if (!xfs_attr_namesp_match(args->flags, sfe->flags))
 			continue;
-		if (args->flags & ATTR_KERNOVAL) {
-			args->valuelen = sfe->valuelen;
-			return 0;
-		}
-		if (args->valuelen < sfe->valuelen) {
-			args->valuelen = sfe->valuelen;
-			return -ERANGE;
-		}
-		args->valuelen = sfe->valuelen;
-		memcpy(args->value, &sfe->nameval[args->namelen],
-						    args->valuelen);
-		return 0;
+		return xfs_attr_copy_value(args, &sfe->nameval[args->namelen],
+						sfe->valuelen);
 	}
 	return -ENOATTR;
 }

@@ -2368,7 +2397,6 @@ xfs_attr3_leaf_getvalue(
 	struct xfs_attr_leaf_entry *entry;
 	struct xfs_attr_leaf_name_local *name_loc;
 	struct xfs_attr_leaf_name_remote *name_rmt;
-	int			valuelen;
 
 	leaf = bp->b_addr;
 	xfs_attr3_leaf_hdr_from_disk(args->geo, &ichdr, leaf);

@@ -2380,18 +2408,9 @@ xfs_attr3_leaf_getvalue(
 		name_loc = xfs_attr3_leaf_name_local(leaf, args->index);
 		ASSERT(name_loc->namelen == args->namelen);
 		ASSERT(memcmp(args->name, name_loc->nameval, args->namelen) == 0);
-		valuelen = be16_to_cpu(name_loc->valuelen);
-		if (args->flags & ATTR_KERNOVAL) {
-			args->valuelen = valuelen;
-			return 0;
-		}
-		if (args->valuelen < valuelen) {
-			args->valuelen = valuelen;
-			return -ERANGE;
-		}
-		args->valuelen = valuelen;
-		memcpy(args->value, &name_loc->nameval[args->namelen], valuelen);
-		return 0;
+		return xfs_attr_copy_value(args,
+					&name_loc->nameval[args->namelen],
+					be16_to_cpu(name_loc->valuelen));
 	}
 
 	name_rmt = xfs_attr3_leaf_name_remote(leaf, args->index);

@@ -2401,16 +2420,7 @@ xfs_attr3_leaf_getvalue(
 	args->rmtblkno = be32_to_cpu(name_rmt->valueblk);
 	args->rmtblkcnt = xfs_attr3_rmt_blocks(args->dp->i_mount,
 					       args->rmtvaluelen);
-	if (args->flags & ATTR_KERNOVAL) {
-		args->valuelen = args->rmtvaluelen;
-		return 0;
-	}
-	if (args->valuelen < args->rmtvaluelen) {
-		args->valuelen = args->rmtvaluelen;
-		return -ERANGE;
-	}
-	args->valuelen = args->rmtvaluelen;
-	return xfs_attr_rmtval_get(args);
+	return xfs_attr_copy_value(args, NULL, args->rmtvaluelen);
 }
 
 /*========================================================================

Signature · 1 line

2.23.0.rc1

From: Dave Chinner <[email protected]>

The same code is used to copy do the attribute copying in three different places. Consolidate them into a single function in preparation from on-demand buffer allocation.

Signed-off-by Dave Chinner <[email protected]>

From: Dave Chinner <[email protected]>

When doing file lookups and checking for permissions, we end up in xfs_get_acl() to see if there are any ACLs on the inode. This requires and xattr lookup, and to do that we have to supply a buffer large enough to hold an maximum sized xattr.

On workloads were we are accessing a wide range of cache cold files under memory pressure (e.g. NFS fileservers) we end up spending a lot of time allocating the buffer. The buffer is 64k in length, so is a contiguous multi-page allocation, and if that then fails we fall back to vmalloc(). Hence the allocation here is /expensive/ when we are looking up hundreds of thousands of files a second.

Initial numbers from a bpf trace show average time in xfs_get_acl() is ~32us, with ~19us of that in the memory allocation. Note these are average times, so there are going to be affected by the worst case allocations more than the common fast case...

To avoid this, we could just do a "null" lookup to see if the ACL xattr exists and then only do the allocation if it exists. This, however, optimises the path for the "no ACL present" case at the expense of the "acl present" case. i.e. we can halve the time in xfs_get_acl() for the no acl case (i.e down to ~10-15us), but that then increases the ACL case by 30% (i.e. up to 40-45us).

To solve this and speed up both cases, drive the xattr buffer allocation into the attribute code once we know what the actual xattr length is. For the no-xattr case, we avoid the allocation completely, speeding up that case. For the common ACL case, we'll end up with a fast heap allocation (because it'll be smaller than a page), and only for the rarer "we have a remote xattr" will we have a multi-page allocation occur. Hence the common ACL case will be much faster, too.

Signed-off-by Dave Chinner <[email protected]>

---
 fs/xfs/libxfs/xfs_attr.c      | 42 ++++++++++++++++++++++++++++++-----
 fs/xfs/libxfs/xfs_attr.h      |  6 +++--
 fs/xfs/libxfs/xfs_attr_leaf.c |  6 +++++
 fs/xfs/libxfs/xfs_da_btree.h  |  4 +++-
 fs/xfs/xfs_acl.c              | 12 ++++------
 fs/xfs/xfs_ioctl.c            |  2 +-
 fs/xfs/xfs_xattr.c            |  2 +-
 7 files changed, 55 insertions(+), 19 deletions(-)

fs/xfs/libxfs/xfs_attr.c +36 -6

index 4773eef9d3de..510ca6974604 100644
@@ -118,12 +118,28 @@ xfs_attr_get_ilocked(
 		return xfs_attr_node_get(args);
 }
 
-/* Retrieve an extended attribute by name, and its value. */
+/*
+ * Retrieve an extended attribute by name, and its value if requested.
+ *
+ * If ATTR_KERNOVAL is set in @flags, then the caller does not want the value,
+ * just an indication whether the attribute exists and the size of the value if
+ * it exists. The size is returned in @valuelenp,
+ *
+ * If the attribute is found, but exceeds the size limit set by the caller in
+ * @valuelenp, return -ERANGE with the size of the attribute that was found in
+ * @valuelenp.
+ *
+ * If ATTR_ALLOC is set in @flags, allocate the buffer for the value after
+ * existence of the attribute has been determined. On success, return that
+ * buffer to the caller and leave them to free it. On failure, free any
+ * allocated buffer and ensure the buffer pointer returned to the caller is
+ * null.
+ */
 int
 xfs_attr_get(
 	struct xfs_inode	*ip,
 	const unsigned char	*name,
-	unsigned char		*value,
+	unsigned char		**value,
 	int			*valuelenp,
 	int			flags)
 {

@@ -131,6 +147,8 @@ xfs_attr_get(
 	uint			lock_mode;
 	int			error;
 
+	ASSERT((flags & (ATTR_ALLOC | ATTR_KERNOVAL)) || *value);
+
 	XFS_STATS_INC(ip->i_mount, xs_attr_get);
 
 	if (XFS_FORCED_SHUTDOWN(ip->i_mount))

@@ -140,17 +158,29 @@ xfs_attr_get(
 	if (error)
 		return error;
 
-	args.value = value;
-	args.valuelen = *valuelenp;
 	/* Entirely possible to look up a name which doesn't exist */
 	args.op_flags = XFS_DA_OP_OKNOENT;
+	if (flags & ATTR_ALLOC)
+		args.op_flags |= XFS_DA_OP_ALLOCVAL;
+	else
+		args.value = *value;
+	args.valuelen = *valuelenp;
 
 	lock_mode = xfs_ilock_attr_map_shared(ip);
 	error = xfs_attr_get_ilocked(ip, &args);
 	xfs_iunlock(ip, lock_mode);
-
 	*valuelenp = args.valuelen;
-	return error;
+
+	/* on error, we have to clean up allocated value buffers */
+	if (error) {
+		if (flags & ATTR_ALLOC) {
+			kmem_free(args.value);
+			*value = NULL;
+		}
+		return error;
+	}
+	*value = args.value;
+	return 0;
 }
 
 /*

fs/xfs/libxfs/xfs_attr.h +4 -2

index ff28ebf3b635..94badfa1743e 100644
@@ -37,6 +37,7 @@ struct xfs_attr_list_context;
 #define ATTR_KERNOVAL	0x2000	/* [kernel] get attr size only, not value */
 
 #define ATTR_INCOMPLETE	0x4000	/* [kernel] return INCOMPLETE attr keys */
+#define ATTR_ALLOC	0x8000	/* allocate xattr buffer on demand */
 
 #define XFS_ATTR_FLAGS \
 	{ ATTR_DONTFOLLOW, 	"DONTFOLLOW" }, \

@@ -47,7 +48,8 @@ struct xfs_attr_list_context;
 	{ ATTR_REPLACE,		"REPLACE" }, \
 	{ ATTR_KERNOTIME,	"KERNOTIME" }, \
 	{ ATTR_KERNOVAL,	"KERNOVAL" }, \
-	{ ATTR_INCOMPLETE,	"INCOMPLETE" }
+	{ ATTR_INCOMPLETE,	"INCOMPLETE" }, \
+	{ ATTR_ALLOC,		"ALLOC" }
 
 /*
  * The maximum size (into the kernel or returned from the kernel) of an

@@ -143,7 +145,7 @@ int xfs_attr_list_int(struct xfs_attr_list_context *);
 int xfs_inode_hasattr(struct xfs_inode *ip);
 int xfs_attr_get_ilocked(struct xfs_inode *ip, struct xfs_da_args *args);
 int xfs_attr_get(struct xfs_inode *ip, const unsigned char *name,
-		 unsigned char *value, int *valuelenp, int flags);
+		 unsigned char **value, int *valuelenp, int flags);
 int xfs_attr_set(struct xfs_inode *dp, const unsigned char *name,
 		 unsigned char *value, int valuelen, int flags);
 int xfs_attr_set_args(struct xfs_da_args *args);

fs/xfs/libxfs/xfs_attr_leaf.c +6 -0

index f6a595e76343..b9f019603d0b 100644
@@ -414,6 +414,12 @@ xfs_attr_copy_value(
 		args->valuelen = valuelen;
 		return -ERANGE;
 	}
+
+	if (args->op_flags & XFS_DA_OP_ALLOCVAL) {
+		args->value = kmem_alloc_large(valuelen, 0);
+		if (!args->value)
+			return -ENOMEM;
+	}
 	args->valuelen = valuelen;
 
 	/* remote block xattr requires IO for copy-in */

fs/xfs/libxfs/xfs_da_btree.h +3 -1

index 84dd865b6c3d..ae0bbd20d9ca 100644
@@ -81,13 +81,15 @@ typedef struct xfs_da_args {
 #define XFS_DA_OP_ADDNAME	0x0004	/* this is an add operation */
 #define XFS_DA_OP_OKNOENT	0x0008	/* lookup/add op, ENOENT ok, else die */
 #define XFS_DA_OP_CILOOKUP	0x0010	/* lookup to return CI name if found */
+#define XFS_DA_OP_ALLOCVAL	0x0020	/* lookup to alloc buffer if found  */
 
 #define XFS_DA_OP_FLAGS \
 	{ XFS_DA_OP_JUSTCHECK,	"JUSTCHECK" }, \
 	{ XFS_DA_OP_RENAME,	"RENAME" }, \
 	{ XFS_DA_OP_ADDNAME,	"ADDNAME" }, \
 	{ XFS_DA_OP_OKNOENT,	"OKNOENT" }, \
-	{ XFS_DA_OP_CILOOKUP,	"CILOOKUP" }
+	{ XFS_DA_OP_CILOOKUP,	"CILOOKUP" }, \
+	{ XFS_DA_OP_ALLOCVAL,	"ALLOCVAL" }
 
 /*
  * Storage for holding state during Btree searches and split/join ops.

fs/xfs/xfs_acl.c +4 -8

index 86c0697870a5..96d7071cfa46 100644
@@ -112,7 +112,7 @@ xfs_get_acl(struct inode *inode, int type)
 {
 	struct xfs_inode *ip = XFS_I(inode);
 	struct posix_acl *acl = NULL;
-	struct xfs_acl *xfs_acl;
+	struct xfs_acl *xfs_acl = NULL;
 	unsigned char *ea_name;
 	int error;
 	int len;

@@ -135,12 +135,8 @@ xfs_get_acl(struct inode *inode, int type)
 	 * go out to the disk.
 	 */
 	len = XFS_ACL_MAX_SIZE(ip->i_mount);
-	xfs_acl = kmem_zalloc_large(len, 0);
-	if (!xfs_acl)
-		return ERR_PTR(-ENOMEM);
-
-	error = xfs_attr_get(ip, ea_name, (unsigned char *)xfs_acl,
-							&len, ATTR_ROOT);
+	error = xfs_attr_get(ip, ea_name, (unsigned char **)&xfs_acl, &len,
+				ATTR_ALLOC | ATTR_ROOT);
 	if (error) {
 		/*
 		 * If the attribute doesn't exist make sure we have a negative

@@ -151,8 +147,8 @@ xfs_get_acl(struct inode *inode, int type)
 	} else  {
 		acl = xfs_acl_from_disk(xfs_acl, len,
 					XFS_ACL_MAX_ENTRIES(ip->i_mount));
+		kmem_free(xfs_acl);
 	}
-	kmem_free(xfs_acl);
 	return acl;
 }
 

fs/xfs/xfs_ioctl.c +1 -1

index 9ea51664932e..6ad63b57b6ca 100644
@@ -438,7 +438,7 @@ xfs_attrmulti_attr_get(
 	if (!kbuf)
 		return -ENOMEM;
 
-	error = xfs_attr_get(XFS_I(inode), name, kbuf, (int *)len, flags);
+	error = xfs_attr_get(XFS_I(inode), name, &kbuf, (int *)len, flags);
 	if (error)
 		goto out_kfree;
 

fs/xfs/xfs_xattr.c +1 -1

index 3123b5aaad2a..cb895b1df5e4 100644
@@ -30,7 +30,7 @@ xfs_xattr_get(const struct xattr_handler *handler, struct dentry *unused,
 		value = NULL;
 	}
 
-	error = xfs_attr_get(ip, (unsigned char *)name, value, &asize, xflags);
+	error = xfs_attr_get(ip, name, (unsigned char **)&value, &asize, xflags);
 	if (error)
 		return error;
 	return asize;

Signature · 1 line

2.23.0.rc1

From: Dave Chinner <[email protected]>

When doing file lookups and checking for permissions, we end up in xfs_get_acl() to see if there are any ACLs on the inode. This requires and xattr lookup, and to do that we have to supply a buffer large enough to hold an maximum sized xattr.

On workloads were we are accessing a wide range of cache cold files under memory pressure (e.g. NFS fileservers) we end up spending a lot of time allocating the buffer. The buffer is 64k in length, so is a contiguous multi-page allocation, and if that then fails we fall back to vmalloc(). Hence the allocation here is /expensive/ when we are looking up hundreds of thousands of files a second.

Initial numbers from a bpf trace show average time in xfs_get_acl() is ~32us, with ~19us of that in the memory allocation. Note these are average times, so there are going to be affected by the worst case allocations more than the common fast case...

To avoid this, we could just do a "null" lookup to see if the ACL xattr exists and then only do the allocation if it exists. This, however, optimises the path for the "no ACL present" case at the expense of the "acl present" case. i.e. we can halve the time in xfs_get_acl() for the no acl case (i.e down to ~10-15us), but that then increases the ACL case by 30% (i.e. up to 40-45us).

To solve this and speed up both cases, drive the xattr buffer allocation into the attribute code once we know what the actual xattr length is. For the no-xattr case, we avoid the allocation completely, speeding up that case. For the common ACL case, we'll end up with a fast heap allocation (because it'll be smaller than a page), and only for the rarer "we have a remote xattr" will we have a multi-page allocation occur. Hence the common ACL case will be much faster, too.

Signed-off-by Dave Chinner <[email protected]>