Re: [PATCH v2 2/3] mm/hmm: allow snapshot of the special zero page

[Jason Gunthorpe <jgg@mellanox.com>]

On Tue, Oct 22, 2019 at 01:06:31PM -0400, Jerome Glisse wrote:

> > > That is fine, the device driver should not do anything with it ie
> > > if the device driver wanted to write then the write fault test
> > > would return true and it would fault.
> > > 
> > > Note that driver should not dereference the struct page.
> > 
> > Can this thing be dma mapped for read?
> > 
> 
> Yes it can, the zero page is just a regular page (AFAIK on all
> architecture). So device can dma map it for read only, there is
> no reason to treat it any differently.
> 
> The HMM_PTE_SPECIAL is only (as documented in the header) for
> pte insert with insert_pfn or insert_page ie pte inserted in
> vma with MIXED or PFNMAP flag. While HMM catch those vma early
> on and backof it can still race with some driver setting the vma
> flag and installing special pte afterward hence why special pte
> goes through this special path.
> 
> The zero page being a special pte is just an exception ie it
> is the only special pte allowed in vma that do not have MIXED or
> PFNMAP flag set.

Just to be clear then, the correct behavior is to return the zero page
pfn as a HMM_PFN_VALID and the driver should treat it the same as any
memory page and dma map it?

Smart drivers can test somehow for pfn == zero_page and optimize?

Jason