linux-kernel - Re: block: DMA alignment of IO buffer allocated from slab

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Date:   Mon, 24 Sep 2018 08:08:26 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ming Lei <ming.lei@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Ming Lei <tom.leiming@...il.com>,
        linux-block <linux-block@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "open list:XFS FILESYSTEM" <linux-xfs@...r.kernel.org>,
        Dave Chinner <dchinner@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Christoph Lameter <cl@...ux.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: block: DMA alignment of IO buffer allocated from slab

On Mon, 2018-09-24 at 17:43 +0300, Andrey Ryabinin wrote:
> 
> On 09/24/2018 05:19 PM, Bart Van Assche wrote:
> > On 9/24/18 2:46 AM, Andrey Ryabinin wrote:
> > > On 09/24/2018 01:42 AM, Ming Lei wrote:
> > > > On Fri, Sep 21, 2018 at 03:04:18PM +0200, Vitaly Kuznetsov wrote:
> > > > > Christoph Hellwig <hch@....de> writes:
> > > > > 
> > > > > > On Wed, Sep 19, 2018 at 05:15:43PM +0800, Ming Lei wrote:
> > > > > > > 1) does kmalloc-N slab guarantee to return N-byte aligned buffer?  If
> > > > > > > yes, is it a stable rule?
> > > > > > 
> > > > > > This is the assumption in a lot of the kernel, so I think if somethings
> > > > > > breaks this we are in a lot of pain.
> > > 
> > > This assumption is not correct. And it's not correct at least from the beginning of the
> > > git era, which is even before SLUB allocator appeared. With CONFIG_DEBUG_SLAB=y
> > > the same as with CONFIG_SLUB_DEBUG_ON=y kmalloc return 'unaligned' objects.
> > > The guaranteed arch-and-config-independent alignment of kmalloc() result is "sizeof(void*)".
> 
> Correction sizeof(unsigned long long), so 8-byte alignment guarantee.
> 
> > > 
> > > If objects has higher alignment requirement, the could be allocated via specifically created kmem_cache.
> > 
> > Hello Andrey,
> > 
> > The above confuses me. Can you explain to me why the following comment is present in include/linux/slab.h?
> > 
> > /*
> >  * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
> >  * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
> >  * aligned pointers.
> >  */
> > 
> 
> ARCH_KMALLOC_MINALIGN - guaranteed alignment of the kmalloc() result.
> ARCH_SLAB_MINALIGN - guaranteed alignment of kmem_cache_alloc() result.
> 
> If the 'align' argument passed into kmem_cache_create() is bigger than ARCH_SLAB_MINALIGN
> than kmem_cache_alloc() from that cache should return 'align'-aligned pointers.

Hello Andrey,

Do you realize that that comment from <linux/slab.h> contradicts what you
wrote about kmalloc() if ARCH_KMALLOC_MINALIGN > sizeof(unsigned long long)?

Additionally, shouldn't CONFIG_DEBUG_SLAB=y and CONFIG_SLUB_DEBUG_ON=y
provide the same guarantees as with debugging disabled, namely that kmalloc()
buffers are aligned on ARCH_KMALLOC_MINALIGN boundaries? Since buffers
allocated with kmalloc() are often used for DMA, how otherwise is DMA assumed
to work?

Thanks,

Bart.