linux-kernel - [PATCH] mm: Skip retry when new limit is not below old one in page_counter_set

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Message-Id: <20220821154055.109635-1-minhquangbui99@gmail.com>
Date:   Sun, 21 Aug 2022 22:40:55 +0700
From:   Bui Quang Minh <minhquangbui99@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Bui Quang Minh <minhquangbui99@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: [PATCH] mm: Skip retry when new limit is not below old one in page_counter_set_max

In page_counter_set_max, we want to make sure the new limit is not below
the concurrently-changing counter value. We read the counter and check that
the limit is not below the counter before the swap. After the swap, we read
the counter again and retry in case the counter is incremented as this may
violate the requirement. Even though the page_counter_try_charge can see
the old limit, it is guaranteed that the counter is not above the old limit
after the increment. So in case the new limit is not below the old limit,
the counter is guaranteed to be not above the new limit too. We can skip
the retry in this case to optimize a little bit.

Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
---
 mm/page_counter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index eb156ff5d603..8a0cc24b60dd 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -193,7 +193,7 @@ int page_counter_set_max(struct page_counter *counter, unsigned long nr_pages)

 		old = xchg(&counter->max, nr_pages);

-		if (page_counter_read(counter) <= usage)
+		if (page_counter_read(counter) <= usage || nr_pages >= old)
 			return 0;

 		counter->max = old;
-- 
2.25.1