Re: [PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk_start()

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Aug 16, 2024 at 10:21:17AM -0400, Peter Xu wrote:
> On Fri, Aug 16, 2024 at 11:30:31AM +0200, David Hildenbrand wrote:
> > On 14.08.24 15:05, Jason Gunthorpe wrote:
> > > On Fri, Aug 09, 2024 at 07:25:36PM +0200, David Hildenbrand wrote:
> > > 
> > > > > > That is in general not what we want, and we still have some places that
> > > > > > wrongly hard-code that behavior.
> > > > > > 
> > > > > > In a MAP_PRIVATE mapping you might have anon pages that we can happily walk.
> > > > > > 
> > > > > > vm_normal_page() / vm_normal_page_pmd() [and as commented as a TODO,
> > > > > > vm_normal_page_pud()] should be able to identify PFN maps and reject them,
> > > > > > no?
> > > > > 
> > > > > Yep, I think we can also rely on special bit.
> > > 
> > > It is more than just relying on the special bit..
> > > 
> > > VM_PFNMAP/VM_MIXEDMAP should really only be used inside
> > > vm_normal_page() because thay are, effectively, support for a limited
> > > emulation of the special bit on arches that don't have them. There are
> > > a bunch of weird rules that are used to try and make that work
> > > properly that have to be followed.
> > > 
> > > On arches with the sepcial bit they should possibly never be checked
> > > since the special bit does everything you need.
> > > 
> > > Arguably any place reading those flags out side of vm_normal_page/etc
> > > is suspect.
> > 
> > IIUC, your opinion matches mine: VM_PFNMAP/VM_MIXEDMAP and pte_special()/...
> > usage should be limited to vm_normal_page/vm_normal_page_pmd/ ... of course,
> > GUP-fast is special (one of the reason for "pte_special()" and friends after
> > all).
> 
> The issue is at least GUP currently doesn't work with pfnmaps, while
> there're potentially users who wants to be able to work on both page +
> !page use cases.  Besides access_process_vm(), KVM also uses similar thing,
> and maybe more; these all seem to be valid use case of reference the vma
> flags for PFNMAP and such, so they can identify "it's pfnmap" or more
> generic issues like "permission check error on pgtable".

Why are those valid compared with calling vm_normal_page() per-page
instead?

What reason is there to not do something based only on the PFNMAP
flag?

Jason