[PATCH v2 6/8] ext4: return -EAGAIN from ext4_map_blocks() in NOWAIT cache miss

[Baokun Li <libaokun@linux.alibaba.com>]

Make ext4_map_blocks() return -EAGAIN instead of 0 when
EXT4_GET_BLOCKS_CACHED_NOWAIT is set and the extent status cache
misses. This allows callers to easily distinguish between a successful
cache lookup (positive return value) and a cache miss requiring disk
access (-EAGAIN), simplifying error handling in NOWAIT paths.

The change affects two locations:
1. After cache hit: return retval ? retval : -EAGAIN
   (return -EAGAIN if cache hit is hole/delayed)
2. After cache miss: return -EAGAIN
   (instead of 0, indicating need for disk lookup)

The only existing caller using EXT4_GET_BLOCKS_CACHED_NOWAIT is the
ext4_get_link() -> ext4_getblk() path. Although ext4_getblk() now
takes a different return branch (err < 0 instead of err == 0) and
propagates -EAGAIN instead of NULL, ext4_get_link() converts both
cases to -ECHILD via IS_ERR_OR_NULL(), so the final error seen by
the VFS remains unchanged.

Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/ext4/inode.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 832794294ccf..03adbca3ec78 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -760,7 +760,8 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		}
 
 		if (flags & EXT4_GET_BLOCKS_CACHED_NOWAIT)
-			return retval;
+			return retval ? retval : -EAGAIN;
+
 #ifdef ES_AGGRESSIVE_TEST
 		ext4_map_blocks_es_recheck(handle, inode, map,
 					   &orig_map, flags);
@@ -776,7 +777,7 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 	 * cannot find extent in the cache.
 	 */
 	if (flags & EXT4_GET_BLOCKS_CACHED_NOWAIT)
-		return 0;
+		return -EAGAIN;
 
 	/*
 	 * Try to see if we can get the block without requesting a new
-- 
2.43.7