Re: [External] Re: [PATCH v13 05/12] mm: hugetlb: allocate the vmemmap pages associated with each HugeTLB page

[Muchun Song]

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
>