Re: [PATCH] LoongArch: Remove shm_align_mask and use SHMLBA instead

[Huacai Chen]

Hi, Xuerui,

On Tue, Aug 29, 2023 at 1:05 AM WANG Xuerui <kernel@xxxxxxxxxx> wrote:
>
> On 8/28/23 23:25, Huacai Chen wrote:
> > Both shm_align_mask and SHMLBA want to avoid cache alias. But they are
> > inconsistent: shm_align_mask is (PAGE_SIZE - 1) while SHMLBA is SZ_64K,
> > but PAGE_SIZE is not always equal to SZ_64K.
> >
> > This may cause problems when shmat() twice. Fix this problem by removing
> > shm_align_mask and using SHMLBA (SHMLBA - 1, strctly) instead.
> "strictly SHMLBA - 1"?
> >
> > Reported-by: Jiantao Shan <shanjiantao@xxxxxxxxxxx>
> > Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > ---
> >   arch/loongarch/mm/cache.c      |  1 -
> >   arch/loongarch/mm/mmap.c       | 12 ++++--------
> >   3 files changed, 8 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/loongarch/mm/cache.c b/arch/loongarch/mm/cache.c
> > index 72685a48eaf0..6be04d36ca07 100644
> > --- a/arch/loongarch/mm/cache.c
> > +++ b/arch/loongarch/mm/cache.c
> > @@ -156,7 +156,6 @@ void cpu_cache_init(void)
> >
> >       current_cpu_data.cache_leaves_present = leaf;
> >       current_cpu_data.options |= LOONGARCH_CPU_PREFETCH;
> > -     shm_align_mask = PAGE_SIZE - 1;
> >   }
> >
> >   static const pgprot_t protection_map[16] = {
> > diff --git a/arch/loongarch/mm/mmap.c b/arch/loongarch/mm/mmap.c
> > index fbe1a4856fc4..c99c8015651a 100644
> > --- a/arch/loongarch/mm/mmap.c
> > +++ b/arch/loongarch/mm/mmap.c
> > @@ -8,12 +8,8 @@
> >   #include <linux/mm.h>
> >   #include <linux/mman.h>
> >
> > -unsigned long shm_align_mask = PAGE_SIZE - 1;        /* Sane caches */
> > -EXPORT_SYMBOL(shm_align_mask);
>
> By removing this altogether, a lot of code duplication is introduced it
> seems. Better make this a private #define so use sites remain nicely
> symbolic:
>
> "#define SHM_ALIGN_MASK (SHMLBA - 1)"
OK, thanks.

Huacai
>
> > -
> > -#define COLOUR_ALIGN(addr, pgoff)                            \
> > -     ((((addr) + shm_align_mask) & ~shm_align_mask) +        \
> > -      (((pgoff) << PAGE_SHIFT) & shm_align_mask))
> > +#define COLOUR_ALIGN(addr, pgoff)                    \
> > +     ((((addr) + (SHMLBA - 1)) & ~(SHMLBA - 1)) + (((pgoff) << PAGE_SHIFT) & (SHMLBA - 1)))
> >
> >   enum mmap_allocation_direction {UP, DOWN};
> >
> > @@ -40,7 +36,7 @@ static unsigned long arch_get_unmapped_area_common(struct file *filp,
> >                * cache aliasing constraints.
> >                */
> >               if ((flags & MAP_SHARED) &&
> > -                 ((addr - (pgoff << PAGE_SHIFT)) & shm_align_mask))
> > +                 ((addr - (pgoff << PAGE_SHIFT)) & (SHMLBA - 1)))
> >                       return -EINVAL;
> >               return addr;
> >       }
> > @@ -63,7 +59,7 @@ static unsigned long arch_get_unmapped_area_common(struct file *filp,
> >       }
> >
> >       info.length = len;
> > -     info.align_mask = do_color_align ? (PAGE_MASK & shm_align_mask) : 0;
> > +     info.align_mask = do_color_align ? (PAGE_MASK & (SHMLBA - 1)) : 0;
> >       info.align_offset = pgoff << PAGE_SHIFT;
> >
> >       if (dir == DOWN) {
>
> --
> WANG "xen0n" Xuerui
>
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>
>