linux-kernel - Re: Re: PG_reserved and compound pages

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Message-ID: <3877205.TjDYue2aah@noys2>
Date:	Wed, 06 Apr 2016 17:12:43 +0200
From:	Frank Mehnert <frank.mehnert@...cle.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Re: PG_reserved and compound pages

Hi Michal,

On Wednesday 06 April 2016 17:02:06 Michal Hocko wrote:
> [CCing linux-mm mailing list]
> 
> On Wed 06-04-16 13:28:37, Frank Mehnert wrote:
> > Hi,
> > 
> > Linux 4.5 introduced additional checks to ensure that compound pages are
> > never marked as reserved. In our code we use PG_reserved to ensure that
> > the kernel does never swap out such pages, e.g.
> 
> Are you putting your pages on the LRU list? If not how they could get
> swapped out?

No, we do nothing like that. It was my understanding that at least with
older kernels it was possible that pages allocated with alloc_pages()
could be swapped out or otherwise manipulated, I might be wrong. For
instance, it's also necessary that the physical address of the page
is known and that it does never change. I know, there might be problems
with automatic NUMA page migration but that's another story.

> >   int i;
> >   struct page *pages = alloc_pages(GFP_HIGHUSER | __GFP_COMP, 4);
> >   for (i = 0; i < 16; i++)
> >   
> >     SetPageReserved(&pages[i]);
> > 
> > The purpose of setting PG_reserved is to prevent the kernel from swapping
> > this memory out. This worked with older kernel but not with Linux 4.5 as
> > setting PG_reserved to compound pages is not allowed any more.
> > 
> > Can somebody explain how we can achieve the same result in accordance to
> > the new Linux 4.5 rules?

Frank
-- 
Dr.-Ing. Frank Mehnert | Software Development Director, VirtualBox
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