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

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Date:   Tue, 7 Feb 2017 10:08:11 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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 7, 2017 at 9:40 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
> 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.

Good question, that's been there since DAX was initially added and I
don't see a good reason for it currently. I'll take a look at what you
have for huge-tmpfs support.