ext4: add nowait mode for ext4_getblk()

Current ext4_getblk() might sleep if some resources are not valid or
could be race with a concurrent extents modifing procedure. So we
cannot call ext4_getblk() and ext4_map_blocks() to get map blocks in
the atomic context in some fast path (e.g. the upcoming procedure of
getting symlink external block in the RCU context), even if the map
extents have already been check and cached.
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://lore.kernel.org/r/20220424140936.1898920-2-yi.zhang@huawei.com

fs/ext4/ext4.h

@@ -673,6 +673,8 @@ enum {
 	/* Caller will submit data before dropping transaction handle. This
 	 * allows jbd2 to avoid submitting data before commit. */
 #define EXT4_GET_BLOCKS_IO_SUBMIT		0x0400
+	/* Caller is in the atomic contex, find extent if it has been cached */
+#define EXT4_GET_BLOCKS_CACHED_NOWAIT		0x0800
 
 /*
  * The bit position of these flags must not overlap with any of the

fs/ext4/inode.c

@@ -545,12 +545,21 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		} else {
 			BUG();
 		}
+
+		if (flags & EXT4_GET_BLOCKS_CACHED_NOWAIT)
+			return retval;
 #ifdef ES_AGGRESSIVE_TEST
 		ext4_map_blocks_es_recheck(handle, inode, map,
 					   &orig_map, flags);
 #endif
 		goto found;
 	}
+	/*
+	 * In the query cache no-wait mode, nothing we can do more if we
+	 * cannot find extent in the cache.
+	 */
+	if (flags & EXT4_GET_BLOCKS_CACHED_NOWAIT)
+		return 0;
 
 	/*
 	 * Try to see if we can get the block without requesting a new
@@ -837,10 +846,12 @@ struct buffer_head *ext4_getblk(handle_t *handle, struct inode *inode,
 	struct ext4_map_blocks map;
 	struct buffer_head *bh;
 	int create = map_flags & EXT4_GET_BLOCKS_CREATE;
+	bool nowait = map_flags & EXT4_GET_BLOCKS_CACHED_NOWAIT;
 	int err;
 
 	ASSERT((EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
 		    || handle != NULL || create == 0);
+	ASSERT(create == 0 || !nowait);
 
 	map.m_lblk = block;
 	map.m_len = 1;
@@ -851,6 +862,9 @@ struct buffer_head *ext4_getblk(handle_t *handle, struct inode *inode,
 	if (err < 0)
 		return ERR_PTR(err);
 
+	if (nowait)
+		return sb_find_get_block(inode->i_sb, map.m_pblk);
+
 	bh = sb_getblk(inode->i_sb, map.m_pblk);
 	if (unlikely(!bh))
 		return ERR_PTR(-ENOMEM);