On Wed, Apr 27, 2022 at 1:27 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 27 Apr 2022 12:58:20 -0700 Peter Collingbourne <pcc@xxxxxxxxxx> wrote:
>
> > When CONFIG_KASAN_HW_TAGS is enabled we currently increase the minimum
> > slab alignment to 16. This happens even if MTE is not supported in
> > hardware or disabled via kasan=off, which creates an unnecessary
> > memory overhead in those cases. Eliminate this overhead by making
> > the minimum slab alignment a runtime property and only aligning to
> > 16 if KASAN is enabled at runtime.
> >
> > On a DragonBoard 845c (non-MTE hardware) with a kernel built with
> > CONFIG_KASAN_HW_TAGS, waiting for quiescence after a full Android
> > boot I see the following Slab measurements in /proc/meminfo (median
> > of 3 reboots):
> >
> > ...
> >
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -3009,10 +3009,9 @@ static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
> > objp += obj_offset(cachep);
> > if (cachep->ctor && cachep->flags & SLAB_POISON)
> > cachep->ctor(objp);
> > - if (ARCH_SLAB_MINALIGN &&
> > - ((unsigned long)objp & (ARCH_SLAB_MINALIGN-1))) {
> > - pr_err("0x%px: not aligned to ARCH_SLAB_MINALIGN=%d\n",
> > - objp, (int)ARCH_SLAB_MINALIGN);
> > + if ((unsigned long)objp & (arch_slab_minalign() - 1)) {
> > + pr_err("0x%px: not aligned to arch_slab_minalign()=%d\n", objp,
> > + (int)arch_slab_minalign());
>
> printf/printk know about size_t. Use %zu, no cast needed. But...
>
> > }
> > return objp;
> > }
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 2b3206a2c3b5..33cc49810a54 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -154,8 +154,7 @@ static unsigned int calculate_alignment(slab_flags_t flags,
> > align = max(align, ralign);
> > }
> >
> > - if (align < ARCH_SLAB_MINALIGN)
> > - align = ARCH_SLAB_MINALIGN;
> > + align = max_t(size_t, align, arch_slab_minalign());
>
> max_t/min_t are nature's way of telling us "you screwed up the types".
>
> So what type _is_ slab alignment? size_t seems sensible, but the code
> prefers unsigned int. So how about we stick with that?
>
>
> This compiles. Still some max_t's in slob.c because I was too lazy to
> go fix the type of ARCH_KMALLOC_MINALIGN.
>
> Shrug, I don't know if we can be bothered. You decide :)
Hi Andrew,
No strong opinions here. I'm happy with the fixup that you added to
your tree on top of my patch.
Peter
