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

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Message-ID: <20160407152234.GE32755@dhcp22.suse.cz>
Date:	Thu, 7 Apr 2016 17:22:35 +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: Re: PG_reserved and compound pages

On Thu 07-04-16 15:45:02, Frank Mehnert wrote:
> On Wednesday 06 April 2016 17:33:43 Michal Hocko wrote:
[...]
> > 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.
> 
> Yes, such memory objects are also mapped to userland. Do you think that
> VM_IO or VM_PFNMAP would guard against NUMA page migration?

Both auto numa and manual numa migration checks vma_migratable and that
excludes both VM flags.

> Because when
> NUMA page migration was introduced (I believe with Linux 3.8) I tested
> both flags and saw that they didn't prevent the migration on such VM
> areas. Maybe this changed in the meantime, do you have more information
> about that?

I haven't checked the history much but vma_migratable should be there
for quite some time. Maybe it wasn't used in the past. Dunno

> The drawback of at least VM_IO is that such memory is not part of a core
> dump.

that seems to be correct as per vma_dump_size

> Actually currently we use vm_insert_page() for userland mapping
> and mark the VM areas as
> 
>   VM_DONTEXPAND | VM_DONTDUMP

but that means that it won't end up in the dump either. Or am I missing
your point.

-- 
Michal Hocko
SUSE Labs