Re: [PATCH V8 5/5] libata: Align DMA buffer to dma_get_cache_alignment()

[Matt Redfearn]

On 18/10/17 14:03, Tejun Heo wrote:
> On Tue, Oct 17, 2017 at 04:05:42PM +0800, Huacai Chen wrote:
> > In non-coherent DMA mode, kernel uses cache flushing operations to
> > maintain I/O coherency, so in ata_do_dev_read_id() the DMA buffer
> > should be aligned to ARCH_DMA_MINALIGN. Otherwise, If a DMA buffer
> > and a kernel structure share a same cache line, and if the kernel
> > structure has dirty data, cache_invalidate (no writeback) will cause
> > data corruption.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Huacai Chen <chenhc@xxxxxxxxxx>
> > ---
> >   drivers/ata/libata-core.c | 15 +++++++++++++--
> >   1 file changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
> > index ee4c1ec..e134955 100644
> > --- a/drivers/ata/libata-core.c
> > +++ b/drivers/ata/libata-core.c
> > @@ -1833,8 +1833,19 @@ static u32 ata_pio_mask_no_iordy(const struct ata_device *adev)
> >   unsigned int ata_do_dev_read_id(struct ata_device *dev,
> >   					struct ata_taskfile *tf, u16 *id)
> >   {
> > -	return ata_exec_internal(dev, tf, NULL, DMA_FROM_DEVICE,
> > -				     id, sizeof(id[0]) * ATA_ID_WORDS, 0);
> > +	u16 *devid;
> > +	int res, size = sizeof(u16) * ATA_ID_WORDS;
> > +
> > +	if (IS_ALIGNED((unsigned long)id, dma_get_cache_alignment(&dev->tdev)))
> > +		res = ata_exec_internal(dev, tf, NULL, DMA_FROM_DEVICE, id, size, 0);
> > +	else {
> > +		devid = kmalloc(size, GFP_KERNEL);
> > +		res = ata_exec_internal(dev, tf, NULL, DMA_FROM_DEVICE, devid, size, 0);
> > +		memcpy(id, devid, size);
> > +		kfree(devid);
> > +	}
> > +
> > +	return res;
> Hmm... I think it'd be a lot better to ensure that the buffers are
> aligned properly to begin with.  There are only two buffers which are
> used for id reading - ata_port->sector_buf and ata_device->id.  Both
> are embedded arrays but making them separately allocated aligned
> buffers shouldn't be difficult.
>
> Thanks.

FWIW, I agree that the buffers used for DMA should be split out from the 
structure. We ran into this problem on MIPS last year, 
4ee34ea3a12396f35b26d90a094c75db95080baa ("libata: Align ata_device's id 
on a cacheline") partially fixed it, but likely should have also 
cacheline aligned the following devslp_timing in the struct such that we 
guarantee that members of the struct not used for DMA do not share the 
same cacheline as the DMA buffer. Not having this means that 
architectures, such as MIPS, which in some cases have to perform manual 
invalidation of DMA buffer can clobber valid adjacent data if it is in 
the same cacheline.

Thanks,
Matt