[PATCH] btrfs: add comment on why we can return 0 if we failed to atomically lock the page in read_extent_buffer_pages()

[Qu Wenruo <wqu@suse.com>]

In read_extent_buffer_pages(), if we failed to lock the page atomically,
we just exit with return value 0.

This is pretty counter-intuitive, as normally if we can't lock what we
need, we would return something like -EAGAIN.

But the that return hides under (wait == WAIT_NONE) branch, which only
get triggered for readahead.

And for readahead, if we failed to lock the page, it means the extent
buffer is either being read by other thread, or has been read and is
under modification.
Either way the eb will or has been cached, thus readahead has no need to
wait for it.

This patch will add extra comment on this counter-intuitive behavior.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/extent_io.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 7f689ad7709c..038adc423454 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5577,6 +5577,13 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num)
 	for (i = 0; i < num_pages; i++) {
 		page = eb->pages[i];
 		if (wait == WAIT_NONE) {
+			/*
+			 * WAIT_NONE is only utilized by readahead. If we can't
+			 * acquire the lock atomically it means either the eb
+			 * is being read out or under modification.
+			 * Either way the eb will be or has been cached,
+			 * readahead can exit safely.
+			 */
 			if (!trylock_page(page))
 				goto unlock_exit;
 		} else {
-- 
2.30.0