linux-kernel - Re: [PATCH v3 2/2] mm, drm/ttm: Fix vm page protection handling

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Date:   Fri, 6 Dec 2019 14:16:10 +0000
From:   Thomas Hellstrom <thellstrom@...are.com>
To:     "mhocko@...nel.org" <mhocko@...nel.org>,
        "thomas_os@...pmail.org" <thomas_os@...pmail.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "christian.koenig@....com" <christian.koenig@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Pv-drivers <Pv-drivers@...are.com>,
        "rcampbell@...dia.com" <rcampbell@...dia.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "jglisse@...hat.com" <jglisse@...hat.com>,
        Linux-graphics-maintainer <Linux-graphics-maintainer@...are.com>
Subject: Re: [PATCH v3 2/2] mm, drm/ttm: Fix vm page protection handling

Hi Michal,

On Fri, 2019-12-06 at 11:30 +0100, Michal Hocko wrote:
> On Fri 06-12-19 09:24:26, Thomas Hellström (VMware) wrote:
> [...]
> > @@ -283,11 +282,26 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct
> > vm_fault *vmf,
> >  			pfn = page_to_pfn(page);
> >  		}
> >  
> > +		/*
> > +		 * Note that the value of @prot at this point may
> > differ from
> > +		 * the value of @vma->vm_page_prot in the caching- and
> > +		 * encryption bits. This is because the exact location
> > of the
> > +		 * data may not be known at mmap() time and may also
> > change
> > +		 * at arbitrary times while the data is mmap'ed.
> > +		 * This is ok as long as @vma->vm_page_prot is not used
> > by
> > +		 * the core vm to set caching- and encryption bits.
> > +		 * This is ensured by core vm using pte_modify() to
> > modify
> > +		 * page table entry protection bits (that function
> > preserves
> > +		 * old caching- and encryption bits), and the @fault
> > +		 * callback being the only function that creates new
> > +		 * page table entries.
> > +		 */
> 
> While this is a very valuable piece of information I believe we need
> to
> document this in the generic code where everybody will find it.
> vmf_insert_mixed_prot sounds like a good place to me. So being
> explicit
> about VM_MIXEDMAP. Also a reference from vm_page_prot to this
> function
> would be really helpeful.
> 
> Thanks!
> 

Just to make sure I understand correctly. You'd prefer this (or
similar) text to be present at the vmf_insert_mixed_prot() and
vmf_insert_pfn_prot() definitions for MIXEDMAP and PFNMAP respectively,
and a pointer from vm_page_prot to that text. Is that correct?

Thanks,
Thomas