linux-kernel - [PATCH v2] mm/swapfile:__swap_duplicate: drop redundant WRITE_ONCE on swap

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Date: Thu, 22 Feb 2024 10:08:45 +1300
From: Barry Song <21cnbao@...il.com>
To: akpm@...ux-foundation.org,
	linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
	Barry Song <v-songbaohua@...o.com>
Subject: [PATCH v2] mm/swapfile:__swap_duplicate: drop redundant WRITE_ONCE on swap_map for err cases

From: Barry Song <v-songbaohua@...o.com>

The code is quite hard to read, we are still writing swap_map after
errors happen. Though the written value is as before,

 has_cache = count & SWAP_HAS_CACHE;
 count &= ~SWAP_HAS_CACHE;
 [snipped]
 WRITE_ONCE(p->swap_map[offset], count | has_cache);

It would be better to entirely drop the WRITE_ONCE for both
performance and readability.

Signed-off-by: Barry Song <v-songbaohua@...o.com>
---
 -v2: drop goto according to Andrew, Thanks!

 mm/swapfile.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 556ff7347d5f..7cb6d9a2d51d 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3320,7 +3320,8 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 	} else
 		err = -ENOENT;			/* unused swap entry */
 
-	WRITE_ONCE(p->swap_map[offset], count | has_cache);
+	if (!err)
+		WRITE_ONCE(p->swap_map[offset], count | has_cache);
 
 unlock_out:
 	unlock_cluster_or_swap_info(p, ci);
-- 
2.34.1