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

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Date:   Mon, 24 Sep 2018 13:54:01 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ming Lei <ming.lei@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        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 13:41 -0700, Matthew Wilcox wrote:
> On Mon, Sep 24, 2018 at 12:56:18PM -0700, Bart Van Assche wrote:
> > On Mon, 2018-09-24 at 11:57 -0700, Matthew Wilcox wrote:
> > > You're not supposed to use kmalloc memory for DMA.  This is why we have
> > > dma_alloc_coherent() and friends.
> > 
> > Are you claiming that all drivers that use DMA should use coherent DMA only? If
> > coherent DMA is the only DMA style that should be used, why do the following
> > function pointers exist in struct dma_map_ops?
> 
> Good job snipping the part of my reply which addressed this.  Go read
> DMA-API.txt yourself.  Carefully.

The snipped part did not contradict your claim that "You're not supposed to use
kmalloc memory for DMA." In the DMA-API.txt document however there are multiple
explicit statements that support allocating memory for DMA with kmalloc(). Here
is one example from the DMA-API.txt section about dma_map_single():

	Not all memory regions in a machine can be mapped by this API.
	Further, contiguous kernel virtual space may not be contiguous as
	physical memory.  Since this API does not provide any scatter/gather
	capability, it will fail if the user tries to map a non-physically
	contiguous piece of memory.  For this reason, memory to be mapped by
	this API should be obtained from sources which guarantee it to be
	physically contiguous (like kmalloc).

Bart.