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

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Date:	Thu, 07 Apr 2016 15:45:02 +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: Re: PG_reserved and compound pages

On Wednesday 06 April 2016 17:33:43 Michal Hocko wrote:
> 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.

OK. It seems to work if I just don't mark these pages as 'PG_reserved'.
Need to do further tests.

> > 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.

Yes, such memory objects are also mapped to userland. Do you think that
VM_IO or VM_PFNMAP would guard against NUMA page migration? 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?

The drawback of at least VM_IO is that such memory is not part of a core
dump. Actually currently we use vm_insert_page() for userland mapping
and mark the VM areas as

  VM_DONTEXPAND | VM_DONTDUMP

for such areas. We used VM_RESERVED for pre-3.7 kernels (old doc says
``VM_RESERVED tells the memory management system not to attempt to swap
out this VMA; it should be set in most device mappings.'' but this didn't
work for 3.7+ anymore.

Thanks,

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