Re: [LSF/MM/BPF TOPIC] HGM for hugetlbfs

[David Hildenbrand <david@redhat.com>]

On 07.06.23 00:40, David Rientjes wrote:
> On Fri, 2 Jun 2023, Mike Kravetz wrote:
> 
>> The benefit of HGM in the case of memory errors is fairly obvious.  As
>> mentioned above, when a memory error is encountered on a hugetlb page,
>> that entire hugetlb page becomes inaccessible to the application.  Losing,
>> 1G or even 2M of data is often catastrophic for an application.  There
>> is often no way to recover.  It just makes sense that recovering from
>> the loss of 4K of data would generally be easier and more likely to be
>> possible.  Today, when Oracle DB encounters a hard memory error on a
>> hugetlb page it will shutdown.  Plans are currently in place repair and
>> recover from such errors if possible.  Isolating the area of data loss
>> to a single 4K page significantly increases the likelihood of repair and
>> recovery.
>>
>> Today, when a memory error is encountered on a hugetlb page an
>> application is 'notified' of the error by a SIGBUS, as well as the
>> virtual address of the hugetlb page and it's size.  This makes sense as
>> hugetlb pages are accessed by a single page table entry, so you get all
>> or nothing.  As mentioned by James above, this is catastrophic for VMs
>> as the hypervisor has just been told that 2M or 1G is now inaccessible.
>> With HGM, we can isolate such errors to 4K.
>>
>> Backing VMs with hugetlb pages is a real use case today.  We are seeing
>> memory errors on such hugetlb pages with the result being VM failures.
>> One of the advantages of backing VMs with THPs is that they are split in
>> the case of memory errors.  HGM would allow similar functionality.
> 
> Thanks for this context, Mike, it's very useful.
> 
> I think everybody is aligned on the desire to map memory at smaller
> granularities for multiple use cases and it's fairly clear that these use
> cases are critically important to multiple stakeholders.
> 
> I think the open question is whether this functionality is supported in
> hugetlbfs (like with HGM) or that there is a hard requirement that we must
> use THP for this support.
> 
> I don't think that hugetlbfs is feature frozen, but if there's a strong
> bias toward not merging additional complexity into the subsystem that
> would useful to know.  I personally think the critical use cases described

At least I, attending that session, thought that it was clear that the 
majority of the people speaking up clearly expressed "no more added 
complexity". So I think there is a clear strong bias, at least from the 
people attending that session.


> above justify the added complexity of HGM to hugetlb and we wouldn't be
> blocked by the long standing (15+ years) desire to mesh hugetlb into the
> core MM subsystem before we can stop the pain associated with memory
> poisoning and live migration.
> 
> Are there strong objections to extending hugetlb for this support?

I don't want to get too involved in this discussion (busy), but I 
absolutely agree on the points that were raised at LSF/MM that

(A) hugetlb is complicated and very special (many things not integrated 
with core-mm, so we need special-casing all over the place). [example: 
what is a pte?]

(B) We added a bunch of complexity in the past that some people 
considered very important (and it was not feature frozen, right? ;) ). 
Looking back, we might just not have done some of that, or done it 
differently/cleaner -- better integrated in the core. (PMD sharing, 
MAP_PRIVATE, a reservation mechanism that still requires preallocation 
because it fails with NUMA/fork, ...)

(C) Unifying hugetlb and the core looks like it's getting more and more 
out of reach, maybe even impossible with all the complexity we added 
over the years (well, and keep adding).

Sure, HGM for the purpose of better hwpoison handling makes sense. But 
hugetlb is probably 20 years old and hwpoison handling probably 13 years 
old. So we managed to get quite far without that optimization.

Absolutely, HGM for better postcopy live migration also makes sense, I 
guess nobody disagrees on that.


But as discussed in that session, maybe we should just start anew and 
implement something that integrates nicely with the core , instead of 
making hugetlb more complicated and even more special.


Now, we all know, nobody wants to do the heavy lifting for that, that's 
why we're discussing how to get in yet another complicated feature.

Maybe we can manage to reduce complexity and integrate some parts nicer 
with core-mm, I don't know.


Don't get me wrong, Mike is the maintainer, I'm just reading along and 
voicing what I observed in the LSF/MM session (well, I mixed in some of 
my own opinion ;) ).

-- 
Cheers,

David / dhildenb