Re: [PATCH] dm-pcache: Fix 64-bit division for 32-bit platforms in get_kset_id()

[Dongsheng Yang <dongsheng.yang@linux.dev>]

在 8/22/2025 4:09 AM, Nathan Chancellor 写道:
> When building for a 32-bit platform (such as ARCH=i386 allmodconfig),
> there is a modpost error:
>
>    ERROR: modpost: "__umoddi3" [drivers/md/dm-pcache/dm-pcache.ko] undefined!
>
> Hacking up the driver Makefile to allow building into the kernel shows
> that the division comes from get_kset_id(), which is inlined into
> cache_key_append() in cache_key.c.
>
> Use the helper div_u64_rem() to avoid emitting a libcall on 32-bit targets.
>
> Fixes: fd5cc4922bef ("dm-pcache: add persistent cache target in device-mapper")
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>   drivers/md/dm-pcache/cache.h | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/dm-pcache/cache.h b/drivers/md/dm-pcache/cache.h
> index b3b361cc406e..b10e721ab1b7 100644
> --- a/drivers/md/dm-pcache/cache.h
> +++ b/drivers/md/dm-pcache/cache.h
> @@ -373,7 +373,9 @@ static inline void *get_key_head_addr(struct pcache_cache *cache)
>   
>   static inline u32 get_kset_id(struct pcache_cache *cache, u64 off)
>   {
> -	return (off >> PCACHE_CACHE_SUBTREE_SIZE_SHIFT) % cache->n_ksets;
> +	u32 rem;
> +	div_u64_rem(off >> PCACHE_CACHE_SUBTREE_SIZE_SHIFT, cache->n_ksets, &rem);
> +	return rem;
>   }


Hi Nathan,

Thanx for your fix. It looks good to me.

Just one nit about the variable name. I prefer kset_id to rem.

Mikulas, if this looks good to you, could you apply this change to dm-6.18?

Thanx

Dongsheng

diff --git a/drivers/md/dm-pcache/cache.h b/drivers/md/dm-pcache/cache.h
index b3b361cc406e..f005c9d9a7aa 100644
--- a/drivers/md/dm-pcache/cache.h
+++ b/drivers/md/dm-pcache/cache.h
@@ -373,7 +373,11 @@ static inline void *get_key_head_addr(struct 
pcache_cache *cache)

  static inline u32 get_kset_id(struct pcache_cache *cache, u64 off)
  {
-       return (off >> PCACHE_CACHE_SUBTREE_SIZE_SHIFT) % cache->n_ksets;
+       u32 kset_id;
+
+       div_u64_rem(off >> PCACHE_CACHE_SUBTREE_SIZE_SHIFT, 
cache->n_ksets, &kset_id);
+
+       return kset_id;
  }

  static inline struct pcache_cache_kset *get_kset(struct pcache_cache 
*cache, u32 kset_id)

> re
>   
>   static inline struct pcache_cache_kset *get_kset(struct pcache_cache *cache, u32 kset_id)
>
> ---
> base-commit: fd5cc4922bef4b3c3cd0452f38dcfd066322e9a9
> change-id: 20250821-dm-pcache-fix-32-bit-div-err-4f0695e94784
>
> Best regards,
> --
> Nathan Chancellor <nathan@kernel.org>
>