linux-kernel - [PATCH] mm/swap: avoid a xa load for swapout path

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Message-ID: <20231017011728.37508-1-ryncsn@gmail.com>
Date:   Tue, 17 Oct 2023 09:17:28 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
        Kairui Song <kasong@...cent.com>
Subject: [PATCH] mm/swap: avoid a xa load for swapout path

From: Kairui Song <kasong@...cent.com>

A variable is never used for swapout path (shadowp is NULL) and compiler
is unable to optimize out the unneeded load since it's a function call.

Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/swap_state.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b3b14bd0dd64..ab79ffb71736 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -109,9 +109,9 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
 			goto unlock;
 		for (i = 0; i < nr; i++) {
 			VM_BUG_ON_FOLIO(xas.xa_index != idx + i, folio);
-			old = xas_load(&xas);
-			if (xa_is_value(old)) {
-				if (shadowp)
+			if (shadowp) {
+				old = xas_load(&xas);
+				if (xa_is_value(old))
 					*shadowp = old;
 			}
 			xas_store(&xas, folio);
-- 
2.42.0