Re: [PATCH v2] scsi: libsas: Allocation SMP request is aligned to ARCH_DMA_MINALIGN

[John Garry]

On 26/03/2024 13:14, Damien Le Moal wrote:
> On 3/26/24 21:43, Yihang Li wrote:
> > This series [1] reducing the kmalloc() minimum alignment on arm64 to 8
> > (from 128). In libsas, this will cause SMP requests to be 8-byte-aligned
> > through kmalloc() allocation. However, for the hisi_sas hardware, all
> > commands address must be 16-byte-aligned. 
> Otherwise, the commands fail to
> > be executed.
> >
> > ARCH_DMA_MINALIGN represents the minimum (static) alignment for safe DMA
> > operations, so use ARCH_DMA_MINALIGN as the alignment for SMP request.
> >
> > Link: https://lkml.kernel.org/r/20230612153201.554742-1-catalin.marinas@xxxxxxx [1]
> > Signed-off-by: Yihang Li <liyihang9@xxxxxxxxxx>
> > ---
> > Changes since v1:
> > - Directly modify alloc_smp_req() instead of using handler callback.
> > ---
> >   drivers/scsi/libsas/sas_expander.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/scsi/libsas/sas_expander.c b/drivers/scsi/libsas/sas_expander.c
> > index a2204674b680..941abc7298df 100644
> > --- a/drivers/scsi/libsas/sas_expander.c
> > +++ b/drivers/scsi/libsas/sas_expander.c
> > @@ -135,7 +135,10 @@ static int smp_execute_task(struct domain_device *dev, void *req, int req_size,
> >   
> >   static inline void *alloc_smp_req(int size)
> >   {
> > -	u8 *p = kzalloc(size, GFP_KERNEL);
> > +	u8 *p;
> > +
> > +	size = ALIGN(size, ARCH_DMA_MINALIGN);


If this is a hisi_sas requirement, then why use ARCH_DMA_MINALIGN and 
not 16B as minimum alignment?

Or are we really talking about an arch requirement?

> > +	p = kzalloc(size, GFP_KERNEL);
>
> Nit: why not:
>
> 	p = kzalloc(ALIGN(size, ARCH_DMA_MINALIGN), GFP_KERNEL);
>
> >   	if (p)
> >   		p[0] = SMP_REQUEST;
> >   	return p;
>
> Otherwise looks OK to me.
>
> John,
>
> Unrelated to this patch, but I wonder if the GFP_KERNEL used here shouldn't be
> GFP_NOIO... Is this ever called in the IO path or error recovery ?

These should not be called in the IO path - as they are management 
functions. But I am quite confident that they can be called in SCSI 
error handling (for libsas).

Thanks,
John