Re: [PATCH 1/1] mm/rmap: optimize MM-ID mapcount handling with union

["Lance Yang" <lance.yang@linux.dev>]

April 20, 2025 at 3:12 PM, "David Hildenbrand" <david@redhat.com> wrote:



> 
> > 
> > /* Note: mapcounts start at -1. */
> > 
> >  atomic_set(&folio->_large_mapcount, mapcount - 1);
> > 
> >  diff --git a/mm/internal.h b/mm/internal.h
> > 
> >  index 838f840ded83..1505174178f4 100644
> > 
> >  --- a/mm/internal.h
> > 
> >  +++ b/mm/internal.h
> > 
> >  @@ -772,8 +772,13 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
> > 
> >  atomic_set(&folio->_nr_pages_mapped, 0);
> > 
> >  if (IS_ENABLED(CONFIG_MM_ID)) {
> > 
> >  folio->_mm_ids = 0;
> > 
> >  - folio->_mm_id_mapcount[0] = -1;
> > 
> >  - folio->_mm_id_mapcount[1] = -1;
> > 
> >  + /*
> > 
> >  + * One-shot initialization of both mapcount slots to -1.
> > 
> >  + * Using 'unsigned long' ensures cross-arch compatibility:
> > 
> >  + * - 32-bit: Fills both short slots (0xFFFF each)
> > 
> >  + * - 64-bit: Fills both int slots (0xFFFFFFFF each)
> > 
> >  + */
> > 
> >  + folio->_mm_id_mapcounts = -1UL;
> > 
> 
> Are we sure the compiler cannot optimize that itself?
> 
> On x86-64 I get with gcc 14.2.1:
> 
> ; folio->_mm_id_mapcount[0] = -1;
> 
>  3f2f: 48 c7 42 60 ff ff ff ff movq $-0x1, 0x60(%rdx)
> 
> Which should be a quadword (64bit) setting, so exactly what you want to achieve.


Yeah, the compiler should be as smart as we expect it to be.
However, it seems that gcc 4.8.5 doesn't behave as expected
with the -O2 optimization level on the x86-64 test machine.

struct folio_array {
    int _mm_id_mapcount[2];
};

void init_array(struct folio_array *f) {
    f->_mm_id_mapcount[0] = -1;
    f->_mm_id_mapcount[1] = -1;
}

0000000000000000 <init_array>:
   0:	c7 07 ff ff ff ff    	movl   $0xffffffff,(%rdi)
   6:	c7 47 04 ff ff ff ff 	movl   $0xffffffff,0x4(%rdi)
   d:	c3                   	retq

---

struct folio_union {
    union {
        int _mm_id_mapcount[2];
        unsigned long _mm_id_mapcounts;
    };
};

void init_union(struct folio_union *f) {
    f->_mm_id_mapcounts = -1UL;
}

0000000000000010 <init_union>:
  10:	48 c7 07 ff ff ff ff 	movq   $0xffffffffffffffff,(%rdi)
  17:	c3                   	retq

Hmm... I'm not sure if it's valuable for those compilers that
are not very new.


Thanks,
Lance

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