ext4: prevent ext4_quota_write() from failing due to ENOSPC

In order to prevent quota block tracking to be inaccurate when
ext4_quota_write() fails with ENOSPC, we make two changes.  The quota
file can now use the reserved block (since the quota file is arguably
file system metadata), and ext4_quota_write() now uses
ext4_should_retry_alloc() to retry the block allocation after a commit
has completed and released some blocks for allocation.

This fixes failures of xfstests generic/270:

Quota error (device vdc): write_blk: dquota write failed
Quota error (device vdc): qtree_write_dquot: Error -28 occurred while creating quota
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/extents.c

@@ -4456,6 +4456,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 		ar.flags |= EXT4_MB_HINT_NOPREALLOC;
 	if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE)
 		ar.flags |= EXT4_MB_DELALLOC_RESERVED;
+	if (flags & EXT4_GET_BLOCKS_METADATA_NOFAIL)
+		ar.flags |= EXT4_MB_USE_RESERVED;
 	newblock = ext4_mb_new_blocks(handle, &ar, &err);
 	if (!newblock)
 		goto out2;

fs/ext4/indirect.c

@@ -576,6 +576,8 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
 		ar.flags = EXT4_MB_HINT_DATA;
 	if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE)
 		ar.flags |= EXT4_MB_DELALLOC_RESERVED;
+	if (flags & EXT4_GET_BLOCKS_METADATA_NOFAIL)
+		ar.flags |= EXT4_MB_USE_RESERVED;
 
 	ar.goal = ext4_find_goal(inode, map->m_lblk, partial);
 

fs/ext4/inode.c

@@ -731,18 +731,18 @@ int ext4_get_block(struct inode *inode, sector_t iblock,
  * `handle' can be NULL if create is zero
  */
 struct buffer_head *ext4_getblk(handle_t *handle, struct inode *inode,
-				ext4_lblk_t block, int create)
+				ext4_lblk_t block, int map_flags)
 {
 	struct ext4_map_blocks map;
 	struct buffer_head *bh;
+	int create = map_flags & EXT4_GET_BLOCKS_CREATE;
 	int err;
 
 	J_ASSERT(handle != NULL || create == 0);
 
 	map.m_lblk = block;
 	map.m_len = 1;
-	err = ext4_map_blocks(handle, inode, &map,
-			      create ? EXT4_GET_BLOCKS_CREATE : 0);
+	err = ext4_map_blocks(handle, inode, &map, map_flags);
 
 	if (err == 0)
 		return create ? ERR_PTR(-ENOSPC) : NULL;
@@ -788,11 +788,11 @@ struct buffer_head *ext4_getblk(handle_t *handle, struct inode *inode,
 }
 
 struct buffer_head *ext4_bread(handle_t *handle, struct inode *inode,
-			       ext4_lblk_t block, int create)
+			       ext4_lblk_t block, int map_flags)
 {
 	struct buffer_head *bh;
 
-	bh = ext4_getblk(handle, inode, block, create);
+	bh = ext4_getblk(handle, inode, block, map_flags);
 	if (IS_ERR(bh))
 		return bh;
 	if (!bh || buffer_uptodate(bh))

fs/ext4/namei.c

@@ -61,7 +61,7 @@ static struct buffer_head *ext4_append(handle_t *handle,
 
 	*block = inode->i_size >> inode->i_sb->s_blocksize_bits;
 
-	bh = ext4_bread(handle, inode, *block, 1);
+	bh = ext4_bread(handle, inode, *block, EXT4_GET_BLOCKS_CREATE);
 	if (IS_ERR(bh))
 		return bh;
 	inode->i_size += inode->i_sb->s_blocksize;

fs/ext4/super.c

@@ -5438,6 +5438,7 @@ static ssize_t ext4_quota_write(struct super_block *sb, int type,
 	struct inode *inode = sb_dqopt(sb)->files[type];
 	ext4_lblk_t blk = off >> EXT4_BLOCK_SIZE_BITS(sb);
 	int err, offset = off & (sb->s_blocksize - 1);
+	int retries = 0;
 	struct buffer_head *bh;
 	handle_t *handle = journal_current_handle();
 
@@ -5458,7 +5459,12 @@ static ssize_t ext4_quota_write(struct super_block *sb, int type,
 		return -EIO;
 	}
 
-	bh = ext4_bread(handle, inode, blk, 1);
+	do {
+		bh = ext4_bread(handle, inode, blk,
+				EXT4_GET_BLOCKS_CREATE |
+				EXT4_GET_BLOCKS_METADATA_NOFAIL);
+	} while (IS_ERR(bh) && (PTR_ERR(bh) == -ENOSPC) &&
+		 ext4_should_retry_alloc(inode->i_sb, &retries));
 	if (IS_ERR(bh))
 		return PTR_ERR(bh);
 	if (!bh)