linux-kernel - [PATCH] mm: page-writeback: simplify memcg handling in test_clear_page

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Message-Id: <20210209214543.112655-1-hannes@cmpxchg.org>
Date:   Tue,  9 Feb 2021 16:45:43 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: [PATCH] mm: page-writeback: simplify memcg handling in test_clear_page_writeback()

Page writeback doesn't hold a page reference, which allows truncate to
free a page the second PageWriteback is cleared. This used to require
special attention in test_clear_page_writeback(), where we had to be
careful not to rely on the unstable page->memcg binding and look up
all the necessary information before clearing the writeback flag.

Since commit 073861ed77b6 ("mm: fix VM_BUG_ON(PageTail) and
BUG_ON(PageWriteback)") test_clear_page_writeback() is called with an
explicit reference on the page, and this dance is no longer needed.

Use unlock_page_memcg() and dec_lruvec_page_stat() directly.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/page-writeback.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index eb34d204d4ee..f6c2c3165d4d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2722,12 +2722,9 @@ EXPORT_SYMBOL(clear_page_dirty_for_io);
 int test_clear_page_writeback(struct page *page)
 {
 	struct address_space *mapping = page_mapping(page);
-	struct mem_cgroup *memcg;
-	struct lruvec *lruvec;
 	int ret;
 
-	memcg = lock_page_memcg(page);
-	lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
+	lock_page_memcg(page);
 	if (mapping && mapping_use_writeback_tags(mapping)) {
 		struct inode *inode = mapping->host;
 		struct backing_dev_info *bdi = inode_to_bdi(inode);
@@ -2755,11 +2752,11 @@ int test_clear_page_writeback(struct page *page)
 		ret = TestClearPageWriteback(page);
 	}
 	if (ret) {
-		dec_lruvec_state(lruvec, NR_WRITEBACK);
+		dec_lruvec_page_state(page, NR_WRITEBACK);
 		dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
 		inc_node_page_state(page, NR_WRITTEN);
 	}
-	__unlock_page_memcg(memcg);
+	unlock_page_memcg(page);
 	return ret;
 }
 
-- 
2.30.0