Re: [PATCH v2 0/9] Add support for shared PTEs across processes

David Hildenbrand
2022-07-18T13:00:31Z

[sorry for not being as responsive as I usually am]

>>
>> They share a *mm* including a consistent virtual memory layout (VMA
>> list). Page table sharing is just a side product of that. You could even
>> call page tables just an implementation detail to produce that
>> consistent virtual memory layout -- described for that MM via a
>> different data structure.
>
> Yes, sharing an mm and vma chain does make it different from implementation point of view.
>
>>
>>> A number of people have commented on potential usefulness of this concept
>>> and implementation.
>>
>> ... and a lot of people raised concerns. Yes, page table sharing to
>> reduce memory consumption/tlb misses/... is something reasonable to
>> have. But that doesn't require mshare, as hugetlb has proven.
>>
>> The design might be useful for a handful of corner (!) cases, but as the
>> cover letter only talks about memory consumption of page tables, I'll
>> not care about those. Once these corner cases are explained and deemed
>> important, we might want to think of possible alternatives to explore
>> the solution space.
>
> Memory consumption by page tables is turning out to be significant issue. I mentioned one real-world example from a
> customer where a 300GB SGA on a 512GB server resulted in OOM when 1500+ processes tried to map parts of the SGA into
> their address space. Some customers are able to solve this issue by switching to hugetlbfs but that is not feasible for
> every one.

Yes. Another use case I am aware of are KVM-based virtual machines, when
VM memory (shmem, file-backed) is not only mapped into the emulator
process, but also into other processes used to carry out I/O (e.g.,
vhost-user).

In that case, it's tempting to simply share the page tables between all
processes for the shared mapping -- automatically, just like
shmem/hugetlb already does.

[...]

>>
>>> suggestion to extend hugetlb PMD sharing was discussed briefly. Conclusion from that discussion and earlier discussion
>>> on mailing list was hugetlb PMD sharing is built with special case code in too many places in the kernel and it is
>>> better to replace it with something more general purpose than build even more on it. Mike can correct me if I got that
>>> wrong.
>>
>> Yes, I pushed for the removal of that yet-another-hugetlb-special-stuff,
>> and asked the honest question if we can just remove it and replace it by
>> something generic in the future. And as I learned, we most probably
>> cannot rip that out without affecting existing user space. Even
>> replacing it by mshare() would degrade existing user space.
>>
>> So the natural thing to reduce page table consumption (again, what this
>> cover letter talks about) for user space (semi- ?)automatically for
>> MAP_SHARED files is to factor out what hugetlb has, and teach generic MM
>> code to cache and reuse page tables (PTE and PMD tables should be
>> sufficient) where suitable.
>>
>> For reasonably aligned mappings and mapping sizes, it shouldn't be too
>> hard (I know, locking ...), to cache and reuse page tables attached to
>> files -- similar to what hugetlb does, just in a generic way. We might
>> want a mechanism to enable/disable this for specific processes and/or
>> VMAs, but these are minor details.
>>
>> And that could come for free for existing user space, because page
>> tables, and how they are handled, would just be an implementation detail.
>>
>>
>> I'd be really interested into what the major roadblocks/downsides
>> file-based page table sharing has. Because I am not convinced that a
>> mechanism like mshare() -- that has to be explicitly implemented+used by
>> user space -- is required for that.
>>
>
> I see two parts to what you are suggesting (please correct me if I get this wrong):
>
> 1. Implement a generic page table sharing mechanism
> 2. Implement a way to use this mechanism from userspace

Yes. Whereby 2) would usually just be some heuristic (e.g.,. file > X
MiB -> start sharing), with an additional way to just disable it or just
enable it. But yes, most of that stuff should just be automatic.

>
> For 1, your suggestion seems to be extract the page table sharing code from hugetlb and make it generic. My approach is
> to create a special mm struct to host the shared page tables and create a minimal set of changes to simply get PTEs from
> this special mm struct whenever a shared VMA is accessed. There may be value to extracting hugetlb page table sharing
> code and recasting it into this framework of special mm struct. I will look some more into it.

The basic idea would be that whenever a MAP_SHARED VMA has a reasonable
size, is aligned in a suitable way (including MAP offset), and
protection match, you can just share PTE tables and even PMD tables. As
page tables of shared mappings usually don't really store per-process
information (exceptions I am aware of are userfaultfd and softdirty
tracking), we can simply share/unshare page tables of shared mappings
fairly easily.

Then, you'd have e.g., 2 sets of page tables cached by the fd that can
be mapped into processes

1) PROT_READ|PROT_WRITE
2) PROT_READ

On VMA protection changes, one would have to unshare (unmap the page
table) and either map another shared one, or map a private one. I don't
think there would be need to optimize e.g., for PROT_NONE, but of
course, other combinations could make sense to cache.

PROT_NONE and other corner cases (softdirty tracking) would simply not
use shared page tables.

Shared page tables would have to be refcounted and one could e.g.,
implement a shrinker that frees unused page tables in the fd cache when
memory reclaim kicks in.

With something like that in place, page table consumption could be
reduced and vmscan/rmap walks could turn out more efficient.

>
> As for 2, is it fair to say you are not fond of explicit opt-in from userspace and would rather have the sharing be file
> based like hugetlb? That is worth considering but is limiting page table sharing to just file objects reasonable? A goal
> for mshare mechanism was to allow shared objects to be files, anonymous pages, RDMA buffers, whatever. Idea being if you
> can map it, you can share it with shared page tables. Maybe that is too ambitious a goal and I am open to course correction.

We can glue it to the file or anything else that's shared I think -- I
don't think we particularly, as long as it's something shared between
processes to be mapped. And to be quite honest, whenever I read about
anonymous memory (i.e., MAP_PRIVATE) I hear my inner voice screaming:
just use *shared* memory when you want to *share* memory between
processes, and optimize that if anything is missing.

Having that said, I understood from previous discussions that there is a
use case of efficient read-only protection across many processes/VMAs. I
was wondering if that could be handled on the fs-level (pte_mkwrite). I
remember I raised the idea before: if one could have a
userfaultfd-wp-style (overlay?) file (system?), user-space could
protect/unprotect file pages via a different mechanism (ioctl) and get
notified about write access via something similar to userfaultfd
user-space handlers, not via signals. Instead of adjusting VMAs, once
would only adjust file page mappings to map the relevant pages R/O when
protecting -- if page tables are shared, that would be efficient.

Now, that is is just a very vague brain dump to get it out of my
(overloaded) system. What I think the overall message is: let's try not
designing new features around page table sharing, let's use page table
sharing as an rmap performance optimization and as a mechanism to reduce
page table overhead. I hope what I said makes any sense, I might eb just
wrong.

--
Thanks,

David / dhildenb