linux-ext4 - [PATCH 1/2] fs: ext4: use BUG_ON if writepage call comes from direct reclaim

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Date:   Tue,  3 Jul 2018 12:11:18 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     mgorman@...hsingularity.net, tytso@....edu,
        adilger.kernel@...ger.ca, darrick.wong@...cle.com,
        dchinner@...hat.com, akpm@...ux-foundation.org
Cc:     yang.shi@...ux.alibaba.com, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] fs: ext4: use BUG_ON if writepage call comes from direct reclaim

direct reclaim doesn't write out filesystem page, only kswapd could do
it. So, if the call comes from direct reclaim, it is definitely a bug.

And, Mel Gormane also mentioned "Ultimately, this will be a BUG_ON." In
commit 94054fa3fca1fd78db02cb3d68d5627120f0a1d4 ("xfs: warn if direct
reclaim tries to writeback pages").

Although it is for xfs, ext4 has the similar behavior, so elevate
WARN_ON to BUG_ON.

And, correct the comment accordingly.

Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: "Theodore Ts'o" <tytso@....edu>
Cc: Andreas Dilger <adilger.kernel@...ger.ca>
Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
---
 fs/ext4/inode.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 2ea07ef..089e388 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2071,7 +2071,7 @@ static int __ext4_journalled_writepage(struct page *page,
  * This function can get called via...
  *   - ext4_writepages after taking page lock (have journal handle)
  *   - journal_submit_inode_data_buffers (no journal handle)
- *   - shrink_page_list via the kswapd/direct reclaim (no journal handle)
+ *   - shrink_page_list via the kswapd (no journal handle)
  *   - grab_page_cache when doing write_begin (have journal handle)
  *
  * We don't do any block allocation in this function. If we have page with
@@ -2148,10 +2148,10 @@ static int ext4_writepage(struct page *page,
 		    (inode->i_sb->s_blocksize == PAGE_SIZE)) {
 			/*
 			 * For memory cleaning there's no point in writing only
-			 * some buffers. So just bail out. Warn if we came here
-			 * from direct reclaim.
+			 * some buffers. So just bail out. It is a bug if we
+			 * came here from direct reclaim.
 			 */
-			WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD))
+			BUG_ON((current->flags & (PF_MEMALLOC|PF_KSWAPD))
 							== PF_MEMALLOC);
 			unlock_page(page);
 			return 0;
-- 
1.8.3.1