linux-kernel - Re: mbind MPOL_INTERLEAVE existing pages

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Message-ID: <20230502163442.GA3873@monkey>
Date:   Tue, 2 May 2023 09:34:42 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Lorenzo Stoakes <lstoakes@...il.com>
Subject: Re: mbind MPOL_INTERLEAVE existing pages

On 05/02/23 15:12, Michal Hocko wrote:
> 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 ;)).

Yes, I was worried about the complexity of such a change.  At a high
level, interleave sounds easy.  But, like most things the details
could add a bunch of complexity.

> > > - 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.

Well, this user has a 'work around'.  They simply make sure to set the
policy of this area (a shared memory segment) before populating.  And,
I don't think they would really be happy with the cost of potentially
migrating hundreds of GB of data.

I'll send out a documentation update.
-- 
Mike Kravetz