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
