[PATCH v4 09/12] btrfs: extent_io: Kill the BUG_ON() in lock_extent_buffer_for_io()

[Qu Wenruo <wqu@suse.com>]

This function needs some extra check on locked pages and eb.

For error handling we need to unlock locked pages and the eb.

Also add comment for possible return values of lock_extent_buffer_for_io().

There is a rare >0 return value branch, where all pages get locked
while write bio is not flushed.

Thankfully it's handled by the only caller, btree_write_cache_pages(),
as later write_one_eb() call will trigger submit_one_bio().
So there shouldn't be any problem.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/extent_io.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index fba5d4a0b194..52c1b0ce6bfd 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3509,19 +3509,25 @@ void wait_on_extent_buffer_writeback(struct extent_buffer *eb)
 		       TASK_UNINTERRUPTIBLE);
 }
 
+/*
+ * Return 0 if nothing went wrong, its pages get locked and submitted.
+ * Return >0 is mostly the same as 0, except bio is not submitted.
+ * Return <0 if something went wrong, no page get locked.
+ */
 static noinline_for_stack int
 lock_extent_buffer_for_io(struct extent_buffer *eb,
 			  struct btrfs_fs_info *fs_info,
 			  struct extent_page_data *epd)
 {
-	int i, num_pages;
+	int i, num_pages, failed_page_nr;
 	int flush = 0;
 	int ret = 0;
 
 	if (!btrfs_try_tree_write_lock(eb)) {
-		flush = 1;
 		ret = flush_write_bio(epd);
-		BUG_ON(ret < 0);
+		if (ret < 0)
+			return ret;
+		flush = 1;
 		btrfs_tree_lock(eb);
 	}
 
@@ -3531,7 +3537,8 @@ lock_extent_buffer_for_io(struct extent_buffer *eb,
 			return 0;
 		if (!flush) {
 			ret = flush_write_bio(epd);
-			BUG_ON(ret < 0);
+			if (ret < 0)
+				return ret;
 			flush = 1;
 		}
 		while (1) {
@@ -3573,7 +3580,10 @@ lock_extent_buffer_for_io(struct extent_buffer *eb,
 		if (!trylock_page(p)) {
 			if (!flush) {
 				ret = flush_write_bio(epd);
-				BUG_ON(ret < 0);
+				if (ret < 0) {
+					failed_page_nr = i;
+					goto err_unlock;
+				}
 				flush = 1;
 			}
 			lock_page(p);
@@ -3581,6 +3591,11 @@ lock_extent_buffer_for_io(struct extent_buffer *eb,
 	}
 
 	return ret;
+err_unlock:
+	/* Unlock these already locked pages */
+	for (i = 0; i < failed_page_nr; i++)
+		unlock_page(eb->pages[i]);
+	return ret;
 }
 
 static void end_extent_buffer_writeback(struct extent_buffer *eb)
-- 
2.20.1