Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH v13 05/12] mm: hugetlb: allocate the vmemmap pages associated with each HugeTLB page | Date | Tue, 26 Jan 2021 16:10:53 +0100 |
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On 26.01.21 15:58, Oscar Salvador wrote: > On Tue, Jan 26, 2021 at 10:36:21AM +0100, David Hildenbrand wrote: >> I think either keep it completely simple (only free vmemmap of hugetlb >> pages allocated early during boot - which is what's not sufficient for >> some use cases) or implement the full thing properly (meaning, solve >> most challenging issues to get the basics running). >> >> I don't want to have some easy parts of complex features merged (e.g., >> breaking other stuff as you indicate below), and later finding out "it's >> not that easy" again and being stuck with it forever. > > Well, we could try to do an optimistic allocation, without tricky loopings. > If that fails, refuse to shrink the pool at that moment. > > The user could always try to shrink it later via /proc/sys/vm/nr_hugepages > interface. > > But I am just thinking out loud..
The real issue seems to be discarding the vmemmap on any memory that has movability constraints - CMA and ZONE_MOVABLE; otherwise, as discussed, we can reuse parts of the thingy we're freeing for the vmemmap. Not that it would be ideal: that once-a-huge-page thing will never ever be a huge page again - but if it helps with OOM in corner cases, sure.
Possible simplification: don't perform the optimization for now with free huge pages residing on ZONE_MOVABLE or CMA. Certainly not perfect: what happens when migrating a huge page from ZONE_NORMAL to (ZONE_MOVABLE|CMA)?
> >>> Of course, this means that e.g: memory-hotplug (hot-remove) will not fully work >>> when this in place, but well. >> >> Can you elaborate? Are we're talking about having hugepages in >> ZONE_MOVABLE that are not migratable (and/or dissolvable) anymore? Than >> a clear NACK from my side. > > Pretty much, yeah.
Note that we most likely soon have to tackle migrating/dissolving (free) hugetlbfs pages from alloc_contig_range() context - e.g., for CMA allocations. That's certainly something to keep in mind regarding any approaches that already break offline_pages().
-- Thanks,
David / dhildenb
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