Re: mbind MPOL_INTERLEAVE existing pages

[Vlastimil Babka <vbabka@suse.cz>]

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 :)

> - Update the documentation to be more explicit?
> 
> I can do either, but just wanted to get opinions before starting.