Re: Sharing page tables across processes (mshare)
From: Khalid Aziz
Date: Wed Nov 01 2023 - 18:41:05 EST
On 11/1/23 08:02, David Hildenbrand wrote:
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What next?
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There were some more discussions on this proposal while I was on
leave for a few months. There is enough interest in this feature to
continue to refine this. I will refine the code further but before
that I want to make sure we have a common understanding of what this
feature should do.
Did you follow-up on the alternatives discussed in a bi-weekly mm session on this topic or is there some other reason
you are leaving that out?
I did a poor job of addressing it :) What we are trying to implement here is to allow disjoint processes to share page
tables AND page protection across all processes. It is not the intent to simply catch a process trying to write to a
protected page. Mechanism already exists for that. The intent is when page protection is changed for one process, it
applies instantly to all processes. Using mprotect to catch a change in page protection continues the same problem
database is experiencing. Database wants to be able to change read/write permissions for terrabytes of data for all
clients very quickly and simultaneously. Today it requires coordination across 1000s of processes to accomplish that. It
is slow and impacts database performance significantly. For each process to have to handle a fault/signal whenever page
protection is changed impacts every process. By sharing same PTE across all processes, any page protection changes apply
instantly to all processes (there is the TLB shootdown issue but as discussed in the meeting, it can be handled). The
mshare proposal implements the instant page protection change while bringing in benefits of shared page tables at the
same time. So the two requirements of this feature are not separable. It is a requirement of the feature to bypass
per-process vma permissions. Processes that require per-process vma permissions would not use mshare and thus the
requirement for a process to opt into mshare. Matthew, your thoughts?
Hopefully I understood your suggestion to separate the two requirements correctly. We can discuss it at another
alignment meeting if that helps.
To be precise, I raised that both problems should likely be decoupled (sharing of page tables as an optimization, NOT
using mprotect to catch write access to pagecache pages). And that page table sharing better remains an implementation
detail.
Sharing of page tables (as learned by hugetlb) can easily be beneficial to other use cases -- for example, multi-process
VMs; no need to bring in mshare. There was the concern that it might not always be reasonable to share page tables, so
one could just have some kind of hint (madvise? mmap flag?) that it might be reasonable to try sharing page tables. But
it would be a pure internal optimization. Just like it is for hugetlb we would unshare as soon as someone does an
mprotect() etc. Initially, you could simply ignore any such hint for filesystems that don't support it. Starting with
shmem sounds reasonable.
Write access to pagecache pages (or also read-access?) would ideally be handled on the pagecache level, so you could
catch any write (page tables, write(), ... and eventually later read access if required) and either notify someone
(uffd-style, just on a fd) or send a signal to the faulting process. That would be a new feature, of course. But we do
have writenotify infrastructure in place to catch write access to pagecache pages already, whereby we inform the FS that
someone wants to write to a PTE-read-only pagecache page.
Once you combine both features, you can easily update only a single shared page table when updating the page protection
as triggered by the FS/yet-to-be-named-feature and have all processes sharing these page tables see the change in one go.
As a result of many discussions, a new distinct version of
original proposal has evolved. Which one do we agree to continue
forward with - (1) current version which restricts sharing to PMD sized
and aligned file mappings only, using just a new mmap flag
(MAP_SHARED_PT), or (2) original version that creates an empty page
table shared mshare region using msharefs and mmap for arbitrary
objects to be mapped into later?
At the meeting Matthew expressed desire to support mapping in any objects in the mshare'd region which makes this
feature much more versatile. That was the intent of the original proposal which was not fd and MMAP_SHARED_PT based.
That is (2) above. The current version was largely based upon your suggestion at LSF/MM to restrict this feature to file
mappings only.
So far my opinion on this is unchanged: turning an implementation detail (sharing of page tables) into a feature to
bypass per-process VMA permissions sounds absolutely bad to me.
I agree if a feature silently bypasses per-process VMA permissions, that is a terrible idea. This is why we have
explicit opt-in requirement and intent is to bypass per-vma permissions by sharing PTE, as opposed to shared PTE
bringing in the feature of bypassing per-vma permissions.
The original concept of mshare certainly sounds interesting, but as discussed a couple of times (LSF/mm), it similarly
sounds "dangerous" the way it was originally proposed.
Having some kind of container that multiple process can mmap (fd?), and *selected* mmap()/mprotect()/ get rerouted to
the container could be interesting; but it might be reasonable to then have separate operations to work on such an fd
(ioctl), and *not* using mmap()/mprotect() for that. And one might only want to allow to mmap that fd with a superset of
all permissions used inside the container (and only MAP_SHARED), and strictly filter what we allow to map into such a
container. page table sharing would likely be an implementation detail.
Just some random thoughts (some of which I previously raised). Probably makes sense to discuss that in a bi-weekly mm
meeting (again, this time with you as well).
I appreciate your thoughts and your helping move this discussion forward.
Thanks,
Khalid