ext4: add ext4_sb_bread_nofail() helper function for ext4_free_branches()

The implicit __GFP_NOFAIL flag in ext4_sb_bread() was removed in commit
8a83ac5 ("ext4: call bdev_getblk() from sb_getblk_gfp()"), meaning
the function can now fail under memory pressure.

Most callers of ext4_sb_bread() propagate the error to userspace and do not
remount the filesystem read-only. However, ext4_free_branches() handles
ext4_sb_bread() failure by remounting the filesystem read-only.

This implies that an ext3 filesystem (mounted via the ext4 driver) could be
forcibly remounted read-only due to a transient page allocation failure,
which is unacceptable.

To mitigate this, introduce a new helper function, ext4_sb_bread_nofail(),
which explicitly uses __GFP_NOFAIL, and use it in ext4_free_branches().

Fixes: 8a83ac5 ("ext4: call bdev_getblk() from sb_getblk_gfp()")
Cc: stable@kernel.org
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: LiBaokun96

fs/ext4/ext4.h

@@ -3134,6 +3134,8 @@ extern struct buffer_head *ext4_sb_bread(struct super_block *sb,
 					 sector_t block, blk_opf_t op_flags);
 extern struct buffer_head *ext4_sb_bread_unmovable(struct super_block *sb,
 						   sector_t block);
+extern struct buffer_head *ext4_sb_bread_nofail(struct super_block *sb,
+						sector_t block);
 extern void ext4_read_bh_nowait(struct buffer_head *bh, blk_opf_t op_flags,
 				bh_end_io_t *end_io, bool simu_fail);
 extern int ext4_read_bh(struct buffer_head *bh, blk_opf_t op_flags,

fs/ext4/indirect.c

@@ -1025,7 +1025,7 @@ static void ext4_free_branches(handle_t *handle, struct inode *inode,
 			}
 
 			/* Go read the buffer for the next level down */
-			bh = ext4_sb_bread(inode->i_sb, nr, 0);
+			bh = ext4_sb_bread_nofail(inode->i_sb, nr);
 
 			/*
 			 * A read failure? Report error and clear slot

fs/ext4/super.c

@@ -265,6 +265,15 @@ struct buffer_head *ext4_sb_bread_unmovable(struct super_block *sb,
 	return __ext4_sb_bread_gfp(sb, block, 0, gfp);
 }
 
+struct buffer_head *ext4_sb_bread_nofail(struct super_block *sb,
+					 sector_t block)
+{
+	gfp_t gfp = mapping_gfp_constraint(sb->s_bdev->bd_mapping,
+			~__GFP_FS) | __GFP_MOVABLE | __GFP_NOFAIL;
+
+	return __ext4_sb_bread_gfp(sb, block, 0, gfp);
+}
+
 void ext4_sb_breadahead_unmovable(struct super_block *sb, sector_t block)
 {
 	struct buffer_head *bh = bdev_getblk(sb->s_bdev, block,