linux-kernel - Re: [PATCH] dax: Split pmd map when fallback on COW

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Date:	Mon, 23 Nov 2015 14:04:24 -0700
From:	Toshi Kani <toshi.kani@....com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Linux MM <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dax: Split pmd map when fallback on COW

On Mon, 2015-11-23 at 12:56 -0800, Dan Williams wrote:
> On Mon, Nov 23, 2015 at 12:45 PM, Toshi Kani <toshi.kani@....com> wrote:
> > On Mon, 2015-11-23 at 12:45 -0800, Dan Williams wrote:
> > > On Mon, Nov 23, 2015 at 12:05 PM, Toshi Kani <toshi.kani@....com> wrote:
> [..]
> > > This is a nop if CONFIG_TRANSPARENT_HUGEPAGE=n, so I don't think it's
> > > a complete fix.
> > 
> > Well, __dax_pmd_fault() itself depends on CONFIG_TRANSPARENT_HUGEPAGE.
> > 
> 
> Indeed it is... I think that's wrong because transparent huge pages
> rely on struct page??

I do not think this issue is related with struct page.  wp_huge_pmd() calls
either do_huge_pmd_wp_page() or dax_pmd_fault().  do_huge_pmd_wp_page() splits a
pmd page when it returns with VM_FAULT_FALLBACK.  So, this change keeps them
consistent on VM_FAULT_FALLBACK.

Thanks,
-Toshi
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