linux-kernel - Re: [PATCH 08/10] mm/slab: Allow dynamic kmalloc() minimum alignment

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Date:   Thu, 7 Apr 2022 18:18:16 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/10] mm/slab: Allow dynamic kmalloc() minimum alignment

On Thu, Apr 07, 2022 at 09:50:23AM +0100, Catalin Marinas wrote:
> On Thu, Apr 07, 2022 at 03:46:37AM +0000, Hyeonggon Yoo wrote:
> > On Tue, Apr 05, 2022 at 02:57:56PM +0100, Catalin Marinas wrote:
> > > --- a/mm/slab_common.c
> > > +++ b/mm/slab_common.c
> > > @@ -838,9 +838,18 @@ void __init setup_kmalloc_cache_index_table(void)
> > >  	}
> > >  }
> > >  
> > > -static void __init
> > > +unsigned int __weak arch_kmalloc_minalign(void)
> > > +{
> > > +	return ARCH_KMALLOC_MINALIGN;
> > > +}
> > > +
> > 
> > As ARCH_KMALLOC_ALIGN and arch_kmalloc_minalign() may not be same after
> > patch 10, I think s/ARCH_KMALLOC_ALIGN/arch_kmalloc_minalign/g
> > for every user of it would be more correct?
> 
> Not if the code currently using ARCH_KMALLOC_MINALIGN needs a constant.
> Yes, there probably are a few places where the code can cope with a
> dynamic arch_kmalloc_minalign() but there are two other cases where a
> constant is needed:
> 
> 1. As a BUILD_BUG check because the code is storing some flags in the
>    bottom bits of a pointer. A smaller ARCH_KMALLOC_MINALIGN works just
>    fine here.
> 
> 2. As a static alignment for DMA requirements. That's where the newly
>    exposed ARCH_DMA_MINALIGN should be used.
> 
> Note that this series doesn't make the situation any worse than before
> since ARCH_DMA_MINALIGN stays at 128 bytes for arm64. Current users can
> evolve to use a dynamic alignment in future patches. My main aim with
> this series is to be able to create kmalloc-64 caches on arm64.

AFAIK there are bunch of drivers that directly calls kmalloc().
It becomes tricky when e.g.) a driver allocates just 32 bytes,
but architecture requires it to be 128-byte aligned.

That's why everything allocated from kmalloc() need to be aligned in
ARCH_DMA_MINALIGN. And It's too hard to update all of drivers
that depends on this fact.

So I'm yet skeptical on decoupling ARCH_DMA/KMALLOC_MINALIGN.
Instead of decoupling it, I'm more into dynamically decreasing it.

Please kindly let me know If I'm missing something ;-)

> > > @@ -851,10 +860,17 @@ new_kmalloc_cache(int idx, enum kmalloc_cache_type type, slab_flags_t flags)
> > >  		flags |= SLAB_ACCOUNT;
> > >  	}
> > >  
> > > -	kmalloc_caches[type][idx] = create_kmalloc_cache(
> > > -					kmalloc_info[idx].name[type],
> > > -					kmalloc_info[idx].size, flags, 0,
> > > -					kmalloc_info[idx].size);
> > > +	if (minalign > ARCH_KMALLOC_MINALIGN) {
> > > +		aligned_size = ALIGN(aligned_size, minalign);
> > > +		aligned_idx = __kmalloc_index(aligned_size, false);
> > > +	}
> > > +
> > > +	if (!kmalloc_caches[type][aligned_idx])
> > > +		kmalloc_caches[type][aligned_idx] = create_kmalloc_cache(
> > > +					kmalloc_info[aligned_idx].name[type],
> > > +					aligned_size, flags, 0, aligned_size);
> > > +	if (idx != aligned_idx)
> > > +		kmalloc_caches[type][idx] = kmalloc_caches[type][aligned_idx];
> > 
> > I would prefer detecting minimum kmalloc size in create_kmalloc_caches()
> > in runtime instead of changing behavior of new_kmalloc_cache().
> 
> That was my initial attempt but we have a couple of
> create_kmalloc_cache() (not *_caches) calls directly, one of them in
> mm/slab.c kmem_cache_init(). So I wanted all the minalign logic in a
> single place, hence I replaced the explicit create_kmalloc_cache() call
> with new_kmalloc_cache(). See this patch and patch 9 for some clean-up.
> 
> -- 
> Catalin

-- 
Thanks,
Hyeonggon