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 | Wed, 27 Jan 2021 11:36:15 +0100 |
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On 26.01.21 16:56, David Hildenbrand wrote: > On 26.01.21 16:34, Oscar Salvador wrote: >> On Tue, Jan 26, 2021 at 04:10:53PM +0100, David Hildenbrand wrote: >>> 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. >> >> Yes, that is one way, but I am not sure how hard would it be to implement. >> Plus the fact that as you pointed out, once that memory is used for vmemmap >> array, we cannot use it again. >> Actually, we would fragment the memory eventually? >> >>> 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)? >> >> But if we do not allow theose pages to be in ZONE_MOVABLE or CMA, there is no >> point in migrate them, right? > > Well, memory unplug "could" still work and migrate them and > alloc_contig_range() "could in the future" still want to migrate them > (virtio-mem, gigantic pages, powernv memtrace). Especially, the latter > two don't work with ZONE_MOVABLE/CMA. But, I mean, it would be fair > enough to say "there are no guarantees for > alloc_contig_range()/offline_pages() with ZONE_NORMAL, so we can break > these use cases when a magic switch is flipped and make these pages > non-migratable anymore". > > I assume compaction doesn't care about huge pages either way, not sure > about numa balancing etc. > > > However, note that there is a fundamental issue with any approach that > allocates a significant amount of unmovable memory for user-space > purposes (excluding CMA allocations for unmovable stuff, CMA is > special): pairing it with ZONE_MOVABLE becomes very tricky as your user > space might just end up eating all kernel memory, although the system > still looks like there is plenty of free memory residing in > ZONE_MOVABLE. I mentioned that in the context of secretmem in a reduced > form as well. > > We theoretically have that issue with dynamic allocation of gigantic > pages, but it's something a user explicitly/rarely triggers and it can > be documented to cause problems well enough. We'll have the same issue > with GUP+ZONE_MOVABLE that Pavel is fixing right now - but GUP is > already known to be broken in various ways and that it has to be treated > in a special way. I'd like to limit the nasty corner cases. > > Of course, we could have smart rules like "don't online memory to > ZONE_MOVABLE automatically when the magic switch is active". That's just > ugly, but could work. >
Extending on that, I just discovered that only x86-64, ppc64, and arm64 really support hugepage migration.
Maybe one approach with the "magic switch" really would be to disable hugepage migration completely in hugepage_migration_supported(), and consequently making hugepage_movable_supported() always return false.
Huge pages would never get placed onto ZONE_MOVABLE/CMA and cannot be migrated. The problem I describe would apply (careful with using ZONE_MOVABLE), but well, it can at least be documented.
-- Thanks,
David / dhildenb
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