linux-kernel - Re: [PATCH] scatterlist: enable sg chaining for all architectures

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Date:	Wed, 29 Apr 2015 09:34:42 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	Christoph Hellwig <hch@....de>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	target-devel@...r.kernel.org,
	Parisc List <linux-parisc@...r.kernel.org>
Subject: Re: [PATCH] scatterlist: enable sg chaining for all architectures

2015-04-29 7:16 GMT+09:00 James Bottomley
<James.Bottomley@...senpartnership.com>:
> On Tue, 2015-04-28 at 14:27 -0700, Andrew Morton wrote:
>> On Sat, 25 Apr 2015 23:56:16 +0900 Akinobu Mita <akinobu.mita@...il.com> wrote:
>>
>> > Some architectures enable sg chaining option while others do not.
>> >
>> > The requirement to enable sg chaining is that pages must be aligned
>> > at a 32-bit boundary in order to overload the LSB of the pointer.
>> > Regardless of whether ARCH_HAS_SG_CHAIN is defined or not, the above
>> > requirement is always chacked by BUG_ON() in sg_assign_page.  So
>> > all architectures can enable sg chaining.
>> >
>> > As you can see from the changes in drivers/target/target_core_rd.c,
>> > enabling SG chaining for all architectures allows us to allocate
>> > discontiguous scatterlist tables which can be traversed throughout
>> > by sg_next() without a special handling for some architectures.
>>
>> Thanks, I'll grab this.  If anyone has concerns, speak now or hold both
>> pieces!
>
> It breaks a host of architectures doesn't it?  I can specifically speak
> for PARISC:  The problem is the way our iommus are consuming
> scatterlists.  They're assuming we can dereference the scatterlist as an
> array (like this code in ccio-dma.c):
>
> static int
> ccio_map_sg(struct device *dev, struct scatterlist *sglist, int nents,
>             enum dma_data_direction direction)
> [...]
>         for(i = 0; i < nents; i++)
>                 prev_len += sglist[i].length;
>
> If you turn on sg chaining on our architecture, we'll run off the end of
> that array dereference and crash.
>
> This can all be fixed by making our architecture dma mapping code use
> iterators instead of array lists, but that needs more code than this
> patch provides.  I assume there are similar issues on a lot of other
> architectures, so before you can contemplate a patch like this, surely
> all the architecture consumers have to be converted to iterator instead
> of array format?
>
> The first place to start would be a survey of who's still using the
> array format.

Agreed.  I could find similar issues in arch/m68k/kernel/dma.c.
(git grep '[^a-z]sg++' shows that there are a lot of similar issues)

Andrew, could you drop this patch from -mm for now?
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