From: "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH] mm: replace FAULT_FLAG_SIZE with parameter to huge_fault
Date: Tue, 7 Feb 2017 11:44:11 +0300
Message-ID: <20170207084411.GA527@node.shutemov.name>
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Cc: Christoph Hellwig <hch@infradead.org>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	linux-xfs@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
	Vlastimil Babka <vbabka@suse.cz>, Jan Kara <jack@suse.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Dan Williams <dan.j.williams@intel.com>
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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.

> >> 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.

-- 
 Kirill A. Shutemov

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