Re: [PATCH] mm: replace FAULT_FLAG_SIZE with parameter to huge_fault

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Feb 07, 2017 at 08:56:56AM -0800, Dan Williams wrote:
> On Tue, Feb 7, 2017 at 12:44 AM, Kirill A. Shutemov
> <kirill@shutemov.name> wrote:
> > On Mon, Feb 06, 2017 at 09:30:22AM -0800, Dan Williams wrote:
> >> On Mon, Feb 6, 2017 at 9:27 AM, Christoph Hellwig <hch@infradead.org> wrote:
> >> > On Mon, Feb 06, 2017 at 08:24:48AM -0800, Dan Williams wrote:
> >> >> > Also can be use this opportunity
> >> >> > to fold ->huge_fault into ->fault?
> >
> > BTW, for tmpfs we already use ->fault for both small and huge pages.
> > If ->fault returned THP, core mm look if it's possible to map the page as
> > huge in this particular VMA (due to size/alignment). If yes mm maps the
> > page with PMD, if not fallback to PTE.
> >
> > I think it would be nice to do the same for DAX: filesystem provides core
> > mm with largest page this part of file can be mapped with (base aligned
> > address + lenght for DAX) and core mm sort out the rest.
> 
> For DAX we would need plumb pfn_t into the core mm so that we have the
> PFN_DEV and PFN_MAP flags beyond the raw pfn.

Sounds good to me.

> >> >> Hmm, yes, just need a scheme to not attempt huge_faults on pte-only handlers.
> >> >
> >> > Do we need anything more than checking vma->vm_flags for VM_HUGETLB?
> >>
> >> s/VM_HUGETLB/VM_HUGEPAGE/
> >>
> >> ...but yes as long as we specify that a VM_HUGEPAGE handler must
> >> minimally handle pud and pmd.
> >
> > VM_HUGEPAGE is result of MADV_HUGEPAGE. It's not required to have THP in
> > the VMA.
> 
> Filesystem-DAX and Device-DAX specify VM_HUGEPAGE by default.

But why? Looks like abuse of the flag.

-- 
 Kirill A. Shutemov