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
