[PATCH v2] mm/swapfile:__swap_duplicate: drop redundant WRITE_ONCE on swap_map for err cases

[PATCH v2] 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>
---
 -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

Re: [PATCH v2] mm/swapfile:__swap_duplicate: drop redundant WRITE_ONCE on swap_map for err cases

[Barry Song]

On Mon, Mar 11, 2024 at 10:37 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 21.02.24 22:08, Barry Song 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.
> >
> > Signed-off-by: Barry Song <v-songbaohua@oppo.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);
>
> Maybe too late, but LGTM
>
> Reviewed-by: David Hildenbrand <david@redhat.com>

David, thanks!  Though it is probably too late as this one has been in
mm-stable for a long time.

>
> --
> Cheers,
>
> David / dhildenb

Thanks
Barry