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

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Message-ID: <20160406153343.GJ24272@dhcp22.suse.cz>
Date:	Wed, 6 Apr 2016 17:33:43 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Frank Mehnert <frank.mehnert@...cle.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Re: PG_reserved and compound pages

On Wed 06-04-16 17:12:43, Frank Mehnert wrote:
> 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.

I do not see anything like that. All the evictable pages should be on
a LRU.

> 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.

Do you map your pages to the userspace? If yes then vma with VM_IO or
VM_PFNMAP should keep any attempt away from those pages.
-- 
Michal Hocko
SUSE Labs