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Date: Sun, 13 Sep 2009 16:46:12 +0100 (BST)
From: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Subject: Re: [PATCH 4/8] mm: FOLL_DUMP replace FOLL_ANON
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To: Minchan Kim <minchan.kim@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mel@csn.ul.ie>, Jeff Chua <jeff.chua.linux@gmail.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Linus Torvalds <torvalds@linux-foundation.org>, Nick Piggin <npiggin@suse.de>, Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Thu, 10 Sep 2009, Minchan Kim wrote:
> > =C2=A0 =C2=A0 =C2=A0 =C2=A0/*
> > =C2=A0 =C2=A0 =C2=A0 =C2=A0 * When core dumping an enormous anonymous a=
rea that nobody
> > - =C2=A0 =C2=A0 =C2=A0 =C2=A0* has touched so far, we don't want to all=
ocate page tables.
> > + =C2=A0 =C2=A0 =C2=A0 =C2=A0* has touched so far, we don't want to all=
ocate unnecessary pages or
> > + =C2=A0 =C2=A0 =C2=A0 =C2=A0* page tables. =C2=A0Return error instead =
of NULL to skip handle_mm_fault,
> > + =C2=A0 =C2=A0 =C2=A0 =C2=A0* then get_dump_page() will return NULL to=
 leave a hole in the dump.
> > + =C2=A0 =C2=A0 =C2=A0 =C2=A0* But we can only make this optimization w=
here a hole would surely
> > + =C2=A0 =C2=A0 =C2=A0 =C2=A0* be zero-filled if handle_mm_fault() actu=
ally did handle it.
> > =C2=A0 =C2=A0 =C2=A0 =C2=A0 */
> > - =C2=A0 =C2=A0 =C2=A0 if (flags & FOLL_ANON) {
> > - =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 page =3D ZERO_PAGE(0=
);
> > - =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 if (flags & FOLL_GET=
)
> > - =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0 get_page(page);
> > - =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 BUG_ON(flags & FOLL_=
WRITE);
> > - =C2=A0 =C2=A0 =C2=A0 }
> > + =C2=A0 =C2=A0 =C2=A0 if ((flags & FOLL_DUMP) &&
> > + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 (!vma->vm_ops || !vma->vm_ops->fau=
lt))
>=20
> How about adding comment about zero page use?

What kind of comment did you have in mind?
We used to use ZERO_PAGE there, but with this patch we're not using it.
I thought the comment above describes what we're doing well enough.

I may have kept too quiet about ZERO_PAGEs, knowing that a later patch
was going to change the story; but I don't see what needs saying here.

Hugh
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