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

From: Akinobu Mita
Date: Tue Apr 28 2015 - 20:34:53 EST

2015-04-29 7:16 GMT+09:00 James Bottomley
> 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@xxxxxxxxx> 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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