Re: [PATCH v3 0/6] add mTHP support for anonymous shmem

[David Hildenbrand <david@redhat.com>]

[...]

>>> How can we use per-block tracking for reclaiming memory and what changes would
>>> be needed? Or is per-block really a non-viable option?
>>
>> The interesting thing is: with mTHP toggles it is opt-in -- like for
>> PMD-sized THP with shmem_enabled -- and we don't have to be that concerned
>> about this problem right now.
> 
> Without respecting the size when allocating large folios, mTHP toggles would
> over allocate. My proposal added earlier to this thread is to combine the 2
> to avoid that case. Otherwise, shouldn't we try to find a solution for the
> reclaiming path?

I think at some point we'll really have to do a better job at reclaiming 
(either memory-overallocation, PUNCHHOLE that couldn't split, but maybe 
also pages that are now all-zero again and could be reclaimed again).

> 
>>
>>>
>>> Clearly, if per-block is viable option, shmem_fault() bug would required to be
>>> fixed first. Any ideas on how to make it reproducible?
>>>
>>> The alternatives discussed where sub-page refcounting and zeropage scanning.
>>
>> Yeah, I don't think sub-page refcounting is a feasible (and certainly not
>> desired ;) ) option in the folio world.
>>
>>> The first one is not possible (IIUC) because of a refactor years back that
>>> simplified the code and also requires extra complexity. The second option would
>>> require additional overhead as we would involve scanning.
>>
>> We'll likely need something similar (scanning, tracking?) for anonymous
>> memory as well. There was a proposal for a THP shrinker some time ago, that
>> would solve part of the problem.
> 
> It's good to know we have the same problem in different places. I'm more
> inclined to keep the information rather than adding an extra overhead. Unless
> the complexity is really overwhelming. Considering the concerns here, not sure
> how much should we try merging with iomap as per Ritesh's suggestion.

As raised in the meeting, I do see value in maintaining the information; 
but I also see why Hugh and Kirill think this might be unwarranted 
complexity in shmem.c. Meaning: we might be able to achieve something 
similar without it, and we don't have to solve the problem right now to 
benefit from large folios.

> 
> The THP shrinker, could you please confirm if it is this following thread?
> 
> https://lore.kernel.org/all/cover.1667454613.git.alexlzhu@fb.com/

Yes, although there was no follow up so far. Possibly, because in the 
current khugepaged approach, there will be a constant back-and-forth 
between khugepaged collapsing memory (and wasting memory in the default 
setting), and the THP shrinker reclaiming memory; doesn't sound quite 
effective :) That needs more thought IMHO.

[...]

>>>> To optimize FALLOC_FL_PUNCH_HOLE for the cases where splitting+freeing
>>>> is not possible at fallcoate() time, detecting zeropages later and
>>>> retrying to split+free might be an option, without per-block tracking.
>>>
>>>>
>>>> (2) mTHP controls
>>>>
>>>> As a default, we should not be using large folios / mTHP for any shmem, just
>>>> like we did with THP via shmem_enabled. This is what this series currently
>>>> does, and is aprt of the whole mTHP user-space interface design.
>>>
>>> That was clear for me too. But what is the reason we want to boot in 'safe
>>> mode'? What are the implications of not respecing that?
>>
>> [...]
>>
>>>
>>> As I understood from the call, mTHP with sysctl knobs is preferred over
>>> optimistic falloc/write allocation? But is still unclear to me why the former
>>> is preferred.
>>
>> I think Hugh's point was that this should be an opt-in feature, just like
>> PMD-sized THP started out, and still is, an opt-in feature.
> 
> I'd be keen to understand the use case for this. Even the current THP controls
> we have in tmpfs. I guess these are just scenarios with no swap involved?
> Are these use cases the same for both tmpfs and shmem anon mm?

Systems without swap are one case I think. The other reason for a toggle 
in the past was to be able to disable it to troubleshoot issues (Hugh 
mentioned in the meeting about unlocking new code paths in shmem.c only 
with a toggle).

Staring at my Fedora system:

$ cat /sys/kernel/mm/transparent_hugepage/shmem_enabled
always within_size advise [never] deny force

Maybe because it uses tmpfs to mount /tmp (interesting article on 
lwn.net about that) and people are concerned about the side-effects 
(that can currently waste memory, or result in more reclaim work being 
required when exceeding file sizes).

For VMs, I know that shmem+THP with memory ballooning is problematic and 
not really recommended.

[...]

>>>
>>>>
>>>> Also, we should properly fallback within the configured sizes, and not jump
>>>> "over" configured sizes. Unless there is a good reason.
>>>>
>>>> (3) khugepaged
>>>>
>>>> khugepaged needs to handle larger folios properly as well. Until fixed,
>>>> using smaller THP sizes as fallback might prohibit collapsing a PMD-sized
>>>> THP later. But really, khugepaged needs to be fixed to handle that.
>>>>
>>>> (4) force/disable
>>>>
>>>> These settings are rather testing artifacts from the old ages. We should not
>>>> add them to the per-size toggles. We might "inherit" it from the global one,
>>>> though.
>>>>
>>>> "within_size" might have value, and especially for consistency, we should
>>>> have them per size.
>>>>
>>>>
>>>>
>>>> So, this series only tackles anonymous shmem, which is a good starting
>>>> point. Ideally, we'd get support for other shmem (especially during fault
>>>> time) soon afterwards, because we won't be adding separate toggles for that
>>>> from the interface POV, and having inconsistent behavior between kernel
>>>> versions would be a bit unfortunate.
>>>>
>>>>
>>>> @Baolin, this series likely does not consider (4) yet. And I suggest we have
>>>> to take a lot of the "anonymous thp" terminology out of this series,
>>>> especially when it comes to documentation.
>>>>
>>>> @Daniel, Pankaj, what are your plans regarding that? It would be great if we
>>>> could get an understanding on the next steps on !anon shmem.
>>>
>>> I realize I've raised so many questions, but it's essential for us to grasp the
>>> mm concerns and usage scenarios. This understanding will provide clarity on the
>>> direction regarding folios for !anon shmem.
>>
>> If I understood correctly, Hugh had strong feelings against not respecting
>> mTHP toggles for shmem. Without per-block tracking, I agree with that.
> 
> My understanding was the same. But I have this follow-up question: should
> we respect mTHP toggles without considering mapping constraints (size and
> index)? Or perhaps we should use within_size where we can fit this intermediate
> approach, as long as mTHP granularity is respected?

Likely we should do exactly the same as we would do with the existing 
PMD-sized THPs.

I recall in the meeting that we discussed that always vs. within_size 
seems to have some value, and that we should respect that setting like 
we did for PMD-sized THP.

Which other mapping constraints could we have?

-- 
Cheers,

David / dhildenb