linux-kernel - Re: [PATCH] mm: fix lockdep issues in writeback handling

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Message-Id: <20250902154043.7214448ff3a9cb68c8d231d5@linux-foundation.org>
Date: Tue, 2 Sep 2025 15:40:43 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Aleksandr Nogikh <nogikh@...gle.com>
Cc: david@...hat.com, joannelkoong@...il.com, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, m.szyprowski@...sung.com,
 mszeredi@...hat.com, willy@...radead.org, syzkaller-bugs@...glegroups.com,
 Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH] mm: fix lockdep issues in writeback handling

On Tue,  2 Sep 2025 11:52:50 +0200 Aleksandr Nogikh <nogikh@...gle.com> wrote:

> Hi,
> 
> When can the patch be expected to reach linux-next?
> Syzbot can't build/boot the tree for more than 12 days already :(

Please don't top-post - it messes things up so much.

There's nothing I can reasonably do about this - it's fixing an issue
that's coming in from Miklos's tree and perhaps he's offline.

Perhaps Stephen can directly add it to linux-next for a while?

From: Marek Szyprowski <m.szyprowski@...sung.com>
To: linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: Marek Szyprowski <m.szyprowski@...sung.com>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, Miklos Szeredi <mszeredi@...hat.com>, Joanne Koong <joannelkoong@...il.com>
Subject: [PATCH] mm: fix lockdep issues in writeback handling
Date: Tue, 26 Aug 2025 15:09:48 +0200
Sender: owner-linux-mm@...ck.org
X-Mailer: git-send-email 2.34.1

Commit 167f21a81a9c ("mm: remove BDI_CAP_WRITEBACK_ACCT") removed
BDI_CAP_WRITEBACK_ACCT flag and refactored code that depend on it.
Unfortunately it also moved some variable intialization out of guarded
scope in writeback handling, what triggers a true lockdep warning. Fix
this by moving initialization to the proper place.

Fixes: 167f21a81a9c ("mm: remove BDI_CAP_WRITEBACK_ACCT")
Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
---
 mm/page-writeback.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 99e80bdb3084..3887ac2e6475 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2984,7 +2984,7 @@ bool __folio_end_writeback(struct folio *folio)
 
 	if (mapping && mapping_use_writeback_tags(mapping)) {
 		struct inode *inode = mapping->host;
-		struct bdi_writeback *wb = inode_to_wb(inode);
+		struct bdi_writeback *wb;
 		unsigned long flags;
 
 		xa_lock_irqsave(&mapping->i_pages, flags);
@@ -2992,6 +2992,7 @@ bool __folio_end_writeback(struct folio *folio)
 		__xa_clear_mark(&mapping->i_pages, folio_index(folio),
 					PAGECACHE_TAG_WRITEBACK);
 
+		wb = inode_to_wb(inode);
 		wb_stat_mod(wb, WB_WRITEBACK, -nr);
 		__wb_writeout_add(wb, nr);
 		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {
@@ -3024,7 +3025,7 @@ void __folio_start_writeback(struct folio *folio, bool keep_write)
 	if (mapping && mapping_use_writeback_tags(mapping)) {
 		XA_STATE(xas, &mapping->i_pages, folio_index(folio));
 		struct inode *inode = mapping->host;
-		struct bdi_writeback *wb = inode_to_wb(inode);
+		struct bdi_writeback *wb;
 		unsigned long flags;
 		bool on_wblist;
 
@@ -3035,6 +3036,7 @@ void __folio_start_writeback(struct folio *folio, bool keep_write)
 		on_wblist = mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK);
 
 		xas_set_mark(&xas, PAGECACHE_TAG_WRITEBACK);
+		wb = inode_to_wb(inode);
 		wb_stat_mod(wb, WB_WRITEBACK, nr);
 		if (!on_wblist) {
 			wb_inode_writeback_start(wb);
-- 
2.34.1