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Date:   Tue, 2 May 2023 15:12:18 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Lorenzo Stoakes <lstoakes@gmail.com>
Subject: Re: mbind MPOL_INTERLEAVE existing pages
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On Tue 02-05-23 09:45:40, Vlastimil Babka wrote:
> On 5/1/23 20:58, Mike Kravetz wrote:
> > I received a question from a customer that was trying to move pages via
> > the mbind system call.  In this specific case, the system had two nodes
> > and all pages in the range were already present on node 0.  They then
> > called mbind with mode MPOL_INTERLEAVE and the MPOL_MF_MOVE_ALL flag.  Their
> > expectation was that half the pages in the range would be moved to node 1
> > in an interleaved pattern.
> > 
> > In the above situation, no pages actually get moved.  This is because mbind
> > creates a list of pages to be moved via:
> > 
> > 	ret = queue_pages_range(mm, start, end, nmask,
> >                           flags | MPOL_MF_INVERT, &pagelist);
> > 
> > No page will be added to the list as queue_folio_required is called for each
> > page to determine if it resides within the set of nodes.  And, all page are
> > within the set.
> > 
> > I have reread the mbind man page several times and agree that one might
> > expect MPOL_INTERLEAVE with MPOL_MF_MOVE_ALL to move pages and create an
> > interleaved pattern.  My question is should we:
> > - Change mbind so that pages are moved to an interleaved pattern?
> 
> I guess it could be worth trying, if there's a use case. And hope nobody
> else is depending on the current behavior and will complain afterwards :)

I am not sure this is worth it wrt. complexity. Essentially it would
require to build up the distribution for the whole range first so 2
passes. Also it could become more tricky if the final node mask has
nodes of difference distances (it would be a reasonable expectation to
distribute withe minimum total distances right ;)).
 
> > - Update the documentation to be more explicit?

Yes, please. I do not think. While this sounds like a neat feature I
think the additional complexity is likely not worth it. A strong usecase
might make a difference though.
-- 
Michal Hocko
SUSE Labs