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

[Nathan Chancellor <nathan@kernel.org>]

Hi Dongsheng,

On Fri, Aug 22, 2025 at 02:53:42PM +0800, Dongsheng Yang wrote:
> 
> 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.

Thanks, seems entirely reasonable, thanks for sending the diff!

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