[PATCH] mm/swapfile:__swap_duplicate: drop redundant WRITE_ONCE on swap_map for err cases

[PATCH] mm/swapfile:__swap_duplicate: drop redundant WRITE_ONCE on swap_map for err cases

[Barry Song]

From: Barry Song <v-songbaohua@oppo.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@oppo.com>
---
 mm/swapfile.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 556ff7347d5f..17557003858c 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3320,6 +3320,9 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 	} else
 		err = -ENOENT;			/* unused swap entry */
 
+	if (err)
+		goto unlock_out;
+
 	WRITE_ONCE(p->swap_map[offset], count | has_cache);
 
 unlock_out:
-- 
2.34.1

Re: [PATCH] mm/swapfile:__swap_duplicate: drop redundant WRITE_ONCE on swap_map for err cases

[Andrew Morton]

On Wed, 21 Feb 2024 22:10:28 +1300 Barry Song <21cnbao@gmail.com> wrote:

> From: Barry Song <v-songbaohua@oppo.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.
> 
> ...
>
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3320,6 +3320,9 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
>  	} else
>  		err = -ENOENT;			/* unused swap entry */
>  
> +	if (err)
> +		goto unlock_out;
> +
>  	WRITE_ONCE(p->swap_map[offset], count | has_cache);
>  
>  unlock_out:

I agree, but why add the goto?

--- a/mm/swapfile.c~mm-swapfile-__swap_duplicate-drop-redundant-write_once-on-swap_map-for-err-cases-fix
+++ a/mm/swapfile.c
@@ -3335,10 +3335,8 @@ static int __swap_duplicate(swp_entry_t
 	} else
 		err = -ENOENT;			/* unused swap entry */
 
-	if (err)
-		goto unlock_out;
-
-	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);
_