On Wed, Jan 27, 2021 at 6:36 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > 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 for your explanation. All thinking seems to be introduced by encountering OOM. :-( In order to move forward and free the hugepage. We should add some restrictions below. 1. Only free the hugepage which is allocated from the ZONE_NORMAL. 2. Disable hugepage migration when this feature is enabled. 3. Using GFP_ATOMIC to allocate vmemmap pages firstly (it can reduce memory fragmentation), if it fails, we use part of the hugepage to remap. Hi Oscar, Mike and David H What's your opinion about this? Should we take this approach? Thanks. > > -- > Thanks, > > David / dhildenb >
