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From:   David Hildenbrand <david@redhat.com>
To:     Oscar Salvador <osalvador@suse.de>
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Organization: Red Hat GmbH
Subject: Re: [PATCH v13 05/12] mm: hugetlb: allocate the vmemmap pages
 associated with each HugeTLB page
Message-ID: <e28399e1-3a24-0f22-b057-76e7c7e70017@redhat.com>
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