Subject: Re: [PATCH 7/8] iommu/intel: Support the gfp argument to the map_pages op

From: Jason Gunthorpe <[email-masked]>
Date: Tue, 17 Jan 2023 09:28:46 -0400

On Tue, Jan 17, 2023 at 03:38:51AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <[email-masked]>
> > Sent: Saturday, January 7, 2023 12:43 AM
> > 
> > @@ -2368,7 +2372,7 @@ static int iommu_domain_identity_map(struct
> > dmar_domain *domain,
> > 
> >  	return __domain_mapping(domain, first_vpfn,
> >  				first_vpfn, last_vpfn - first_vpfn + 1,
> > -				DMA_PTE_READ|DMA_PTE_WRITE);
> > +				DMA_PTE_READ|DMA_PTE_WRITE,
> > GFP_KERNEL);
> >  }
> 
> Baolu, can you help confirm whether switching from GFP_ATOMIC to
> GFP_KERNEL is OK in this path? it looks fine to me in a quick glance
> but want to be conservative here.

I checked it carefully myself as well, good to check again.

> > @@ -4333,7 +4337,8 @@ static size_t intel_iommu_unmap(struct
> > iommu_domain *domain,
> > 
> >  	/* Cope with horrid API which requires us to unmap more than the
> >  	   size argument if it happens to be a large-page mapping. */
> > -	BUG_ON(!pfn_to_dma_pte(dmar_domain, iova >> VTD_PAGE_SHIFT,
> > &level));
> > +	BUG_ON(!pfn_to_dma_pte(dmar_domain, iova >> VTD_PAGE_SHIFT,
> > &level,
> > +			       GFP_ATOMIC));
> 
> with level==0 it implies it's only lookup w/o pgtable allocation. From this
> angle it reads better to use a more relaxed gfp e.g. GFP_KERNEL here.

We should only write GFP_KERNEL if it is actually a sleepable context
because it will be mighty confusing if it isn't. I couldn't tell what
the context is so I left it as ATOMIC.

You are correct this is only just a lookup and so the value is never
used / doesn't matter.

Jason