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

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Date:   Tue, 7 Feb 2017 08:56:56 -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 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.

>
>> >> 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.