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

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

[Kairui Song]

From: Kairui Song <kasong@tencent.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@tencent.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

Re: [PATCH] mm/swap: avoid a xa load for swapout path

[Matthew Wilcox]

On Tue, Oct 17, 2023 at 09:17:28AM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.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@tencent.com>

Reviewed-by: Matthew Wilcox (Oracle)

Possibly worth adding a reference to
3852f6768ede ("mm/swapcache: support to handle the shadow entries")

Not sure it's worth a Fixes:, but that's where it was introduced.