linux-kernel - Re: PG_reserved and compound pages

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Message-ID: <20160406150206.GB24283@dhcp22.suse.cz>
Date:	Wed, 6 Apr 2016 17:02:06 +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: PG_reserved and compound pages

[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?

> 
>   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?
> 
> Thanks,
> 
> Frank
-- 
Michal Hocko
SUSE Labs