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

[David Hildenbrand]

[...]

> > > 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@xxxxxx/

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