From 2c4d8cb737b805ca8d890e50c23f2b5eca270733 Mon Sep 17 00:00:00 2001 From: Qu Wenruo Date: Thu, 28 Jan 2021 19:25:08 +0800 Subject: [PATCH] btrfs: explain page locking and readahead in read_extent_buffer_pages() In read_extent_buffer_pages(), if we failed to lock the page atomically, we just exit with return value 0. This is counter-intuitive, as normally if we can't lock what we need, we would return something like EAGAIN. But that return hides under (wait == WAIT_NONE) branch, which only gets 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. Add comment on this counter-intuitive behavior. Reported-by: Dan Carpenter Reviewed-by: Filipe Manana Signed-off-by: Qu Wenruo Signed-off-by: David Sterba --- 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 40d3bca6aaa4c..4be117adda335 100644 --- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -5853,6 +5853,13 @@ 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 { -- 2.30.2