xfs: convert big array and blob array to use memfd backendrepair-part-one_2019-04-15

There are several problems with the initial implementations of the big
array and the blob array data structures.  First, using linked lists
imposes a two-pointer overhead on every record stored.  For blobs this
isn't serious, but for fixed-size records this increases memory
requirements by 40-60%.  Second, we're using kernel memory to store the
intermediate records.  Kernel memory cannot be paged out, which means we
run the risk of OOMing the machine when we run out of physical memory.

Therefore, replace the linked lists in both structures with memfd files.
Random access becomes much easier, memory overhead drops to a negligible
amount, and because memfd pages can be swapped, we have considerably
more flexibility for memory use.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 59 insertions, 35 deletions

diff --git a/fs/xfs/scrub/blob.c b/fs/xfs/scrub/blob.c
index 7113e14a90fc..dbae83ca390c 100644
--- a/fs/xfs/scrub/blob.c
+++ b/fs/xfs/scrub/blob.c
@@ -8,38 +8,48 @@
 #include "xfs_shared.h"
 #include "scrub/array.h"
 #include "scrub/blob.h"
+#include "scrub/xfile.h"
 
 /*
  * XFS Blob Storage
  * ================
- * Stores and retrieves blobs using a list.  Objects are appended to
- * the list and the pointer is returned as a magic cookie for retrieval.
+ * Stores and retrieves blobs using a memfd object.  Objects are appended to
+ * the file and the offset is returned as a magic cookie for retrieval.
  */
 
 #define XB_KEY_MAGIC	0xABAADDAD
 struct xb_key {
-	struct list_head	list;
 	uint32_t		magic;
 	uint32_t		size;
+	loff_t			offset;
 	/* blob comes after here */
 } __attribute__((packed));
 
-#define XB_KEY_SIZE(sz)	(sizeof(struct xb_key) + (sz))
-
 /* Initialize a blob storage object. */
 struct xblob *
 xblob_init(void)
 {
 	struct xblob	*blob;
+	struct file	*filp;
 	int		error;
 
+	filp = xfile_create("blob storage");
+	if (!filp)
+		return ERR_PTR(-ENOMEM);
+	if (IS_ERR(filp))
+		return ERR_CAST(filp);
+
 	error = -ENOMEM;
 	blob = kmem_alloc(sizeof(struct xblob), KM_NOFS | KM_MAYFAIL);
 	if (!blob)
-		return ERR_PTR(error);
+		goto out_filp;
 
-	INIT_LIST_HEAD(&blob->list);
+	blob->filp = filp;
+	blob->last_offset = PAGE_SIZE;
 	return blob;
+out_filp:
+	fput(filp);
+	return ERR_PTR(error);
 }
 
 /* Destroy a blob storage object. */
@@ -47,12 +57,7 @@ void
 xblob_destroy(
 	struct xblob	*blob)
 {
-	struct xb_key	*key, *n;
-
-	list_for_each_entry_safe(key, n, &blob->list, list) {
-		list_del(&key->list);
-		kmem_free(key);
-	}
+	xfile_destroy(blob->filp);
 	kmem_free(blob);
 }
 
@@ -64,19 +69,24 @@ xblob_get(
 	void		*ptr,
 	uint32_t	size)
 {
-	struct xb_key	*key = (struct xb_key *)cookie;
+	struct xb_key	key;
+	loff_t		pos = cookie;
+	int		error;
+
+	error = xfile_io(blob->filp, XFILE_IO_READ, &pos, &key, sizeof(key));
+	if (error)
+		return error;
 
-	if (key->magic != XB_KEY_MAGIC) {
+	if (key.magic != XB_KEY_MAGIC || key.offset != cookie) {
 		ASSERT(0);
 		return -ENODATA;
 	}
-	if (size < key->size) {
+	if (size < key.size) {
 		ASSERT(0);
 		return -EFBIG;
 	}
 
-	memcpy(ptr, key + 1, key->size);
-	return 0;
+	return xfile_io(blob->filp, XFILE_IO_READ, &pos, ptr, key.size);
 }
 
 /* Store a blob. */
@@ -87,19 +97,28 @@ xblob_put(
 	void		*ptr,
 	uint32_t	size)
 {
-	struct xb_key	*key;
-
-	key = kmem_alloc(XB_KEY_SIZE(size), KM_NOFS | KM_MAYFAIL);
-	if (!key)
-		return -ENOMEM;
-
-	INIT_LIST_HEAD(&key->list);
-	list_add_tail(&key->list, &blob->list);
-	key->magic = XB_KEY_MAGIC;
-	key->size = size;
-	memcpy(key + 1, ptr, size);
-	*cookie = (xblob_cookie)key;
+	struct xb_key	key = {
+		.offset = blob->last_offset,
+		.magic = XB_KEY_MAGIC,
+		.size = size,
+	};
+	loff_t		pos = blob->last_offset;
+	int		error;
+
+	error = xfile_io(blob->filp, XFILE_IO_WRITE, &pos, &key, sizeof(key));
+	if (error)
+		goto out_err;
+
+	error = xfile_io(blob->filp, XFILE_IO_WRITE, &pos, ptr, size);
+	if (error)
+		goto out_err;
+
+	*cookie = blob->last_offset;
+	blob->last_offset = pos;
 	return 0;
+out_err:
+	xfile_discard(blob->filp, blob->last_offset, pos - 1);
+	return -ENOMEM;
 }
 
 /* Free a blob. */
@@ -108,14 +127,19 @@ xblob_free(
 	struct xblob	*blob,
 	xblob_cookie	cookie)
 {
-	struct xb_key	*key = (struct xb_key *)cookie;
+	struct xb_key	key;
+	loff_t		pos = cookie;
+	int		error;
+
+	error = xfile_io(blob->filp, XFILE_IO_READ, &pos, &key, sizeof(key));
+	if (error)
+		return error;
 
-	if (key->magic != XB_KEY_MAGIC) {
+	if (key.magic != XB_KEY_MAGIC || key.offset != cookie) {
 		ASSERT(0);
 		return -ENODATA;
 	}
-	key->magic = 0;
-	list_del(&key->list);
-	kmem_free(key);
+
+	xfile_discard(blob->filp, cookie, cookie + sizeof(key) + key.size - 1);
 	return 0;
 }