linux-kernel - Re: [PATCHv4 33/39] thp, mm: implement do_huge_linear

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Date:	Wed, 22 May 2013 18:26:15 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Dave Hansen <dave@...1.net>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 33/39] thp, mm: implement do_huge_linear_fault()

Hillf Danton wrote:
> On Sun, May 12, 2013 at 9:23 AM, Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> >         page = vmf.page;
> > +
> > +       /*
> > +        * If we asked for huge page we expect to get it or VM_FAULT_FALLBACK.
> > +        * If we don't ask for huge page it must be splitted in ->fault().
> > +        */
> > +       BUG_ON(PageTransHuge(page) != thp);
> > +
> Based on the log message in 34/39(
> If the area of page cache required to create huge is empty, we create a
> new huge page and return it.), the above trap looks bogus.

The statement in 34/39 is true for (flags & FAULT_FLAG_TRANSHUGE).
For !(flags & FAULT_FLAG_TRANSHUGE) huge page must be split in ->fault.

The BUG_ON() above is shortcut for two checks:

if (thp)
	BUG_ON(!PageTransHuge(page));
if (!thp)
	BUG_ON(PageTransHuge(page));

-- 
 Kirill A. Shutemov
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