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Date: Tue, 25 Sep 2018 08:20:58 +0800
From: Ming Lei <ming.lei@redhat.com>
To: Christopher Lameter <cl@linux.com>
Cc: Bart Van Assche <bvanassche@acm.org>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Christoph Hellwig <hch@lst.de>, Ming Lei <tom.leiming@gmail.com>,
	linux-block <linux-block@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	"open list:XFS FILESYSTEM" <linux-xfs@vger.kernel.org>,
	Dave Chinner <dchinner@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Jens Axboe <axboe@kernel.dk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: block: DMA alignment of IO buffer allocated from slab
Message-ID: <20180925002057.GB14386@ming.t460p>
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On Mon, Sep 24, 2018 at 03:17:16PM +0000, Christopher Lameter wrote:
> On Mon, 24 Sep 2018, Bart Van Assche wrote:
> 
> > /*
> >  * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
> >  * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
> >  * aligned pointers.
> >  */
> 
> kmalloc alignment is only guaranteed to ARCH_KMALLOC_MINALIGN. That power
> of 2 byte caches (without certain options) are aligned to the power of 2
> is due to the nature that these objects are stored in SLUB. Other
> allocators may behave different and actually different debug options
> result in different alignments. You cannot rely on that.
> 
> ARCH_KMALLOC minalign shows the mininum alignment guarantees. If that is
> not sufficient and you do not want to change the arch guarantees then you
> can open you own slab cache with kmem_cache_create() where you can specify
> different alignment requirements.

Christopher, thank you for clarifying the point!

Then looks it should be reasonable for XFS to switch to kmem_cache_create()
for addressing this issue.

Thanks,
Ming