linux-kernel - Re: [PATCH] mm: softdirty: unmapped addresses between VMAs are clean

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Message-Id: <20140911125436.51338d98e8e84abacc418aff@linux-foundation.org>
Date:	Thu, 11 Sep 2014 12:54:36 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Feiner <pfeiner@...gle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Jamie Liu <jamieliu@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH] mm: softdirty: unmapped addresses between VMAs are
 clean

On Wed, 10 Sep 2014 22:41:04 -0700 Peter Feiner <pfeiner@...gle.com> wrote:

> On Wed, Sep 10, 2014 at 04:36:28PM -0700, Andrew Morton wrote:
> > On Wed, 10 Sep 2014 16:24:46 -0700 Peter Feiner <pfeiner@...gle.com> wrote:
> > > @@ -1048,32 +1048,51 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> > > +	while (1) {
> > > +		unsigned long vm_start = end;
> > 
> > Did you really mean to do that?  If so, perhaps a little comment to
> > explain how it works?
> 
> It's the same idea that I used in the pagemap_pte_hole patch that I submitted
> today: if vma is NULL, then we fill in the pagemap from (addr) to (end) with
> non-present pagemap entries. 
> 
> Should I submit a v2 with a comment?

I spent quite some time staring at that code wondering wtf, so anything
you can do to clarify it would be good.

I think a better name would be plain old "start", to communicate that
it's just a local convenience variable.  "vm_start" means "start of a
vma" and that isn't accurate in this context; in fact it is misleading.

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