2024-02-21 09:13:08

by Barry Song

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

From: Barry Song <[email protected]>

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 <[email protected]>
---
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

2024-02-21 20:36:16

by Andrew Morton

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

On Wed, 21 Feb 2024 22:10:28 +1300 Barry Song <[email protected]> wrote:

> From: Barry Song <[email protected]>
>
> 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);
_