linux-ext4 - Re: [PATCH] mm: replace FAULT_FLAG_SIZE with parameter to huge

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Date:   Tue, 7 Feb 2017 20:40:42 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm: replace FAULT_FLAG_SIZE with parameter to huge_fault

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@...temov.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@...radead.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