Re: [PATCH RFC 0/4] mm: Enable PM_SWAP for shmem with PTE_MARKER

From: David Hildenbrand
Date: Tue Aug 17 2021 - 05:04:28 EST

Hi Peter, a couple of comments, sorry for the late reply.


[Based on v5.14-rc4]

This patchset enables PM_SWAP of pagemap on shmem. IOW userspace will be able
to detect whether a shmem page is swapped out, just like anonymous pages.

This feature can be enabled with CONFIG_PTE_MARKER_PAGEOUT. When enabled, it
brings 0.8% overhead on swap-in performance on a shmem page, so I didn't make
it the default yet. However IMHO 0.8% is still in an acceptable range that we
can even make it the default at last. Comments are welcomed here.

Special config option and added complexity for handling a corner case feature partially more correct. Hm.

There's one previous series that wanted to address the same issue but in
another way by Tiberiu A Georgescu <tiberiu.georgescu@xxxxxxxxxxx>, here:

In that series it's done by looking up page cache for all none ptes. However I
raised concern on 4x performance degradation for all shmem pagemap users.

Who cares? I am asking because for me, it's hard to imagine a workload that actually cares about a 4x performance degradation when querying the pagemap in very special cases, especially if it involves gigantic shmem ranges. VM live migration -- sync will be a bit slower? CRIU sync will be a bit slower? I mean, actual page dumping is a lot more expensive. Really a problem?

I read that CRIU cares about shmem via pagemap [1], at least for anonymous shared memory; not sure how memfd is treated, I assume in a similar way. But I do wonder how it even works reliably, because it relies on present/swapped out and sofrtdirty tracking, which are both essentially broken e.g., when swapping out AFAIKT. Looks like this really needs a proper fix.


Unlike the other approach, this series has zero overhead on pagemap read
because the PM_SWAP info is consolidated into the zapped PTEs directly.


One major goal of this series is to add the PM_SWAP support, the reason is as
stated by Tiberiu and Ivan in the other patchset:

As a summary: for some reason the userspace needs to scan the pages in the
background, however that scanning could misguide page reclaim on which page is
hot and which is cold. With correct PM_SWAP information, the userspace can
correct the behavior of page reclaim by firstly fetching that info from
pagemap, and explicit madvise(MADV_PAGEOUT). In this case, the pages are for
the guest, but it can be any shmem page.

Another major goal of this series is to do a proof-of-concept of the PTE marker
idea, and that's also the major reason why it's RFC. So far PTE marker can
potentially be the solution for below three problems that I'm aware of:

(a) PM_SWAP on shmem

(b) Userfaultfd-wp on shmem/hugetlbfs

(c) PM_SOFT_DIRTY lost for shmem over swapping

This series tries to resolve problem (a) which should be the simplest, ideally
it should solve immediate problem for the live migration issue raised by
Tiberiu and Ivan on proactive paging out unused guest pages.

Both (a) and (c) will be for performance-wise or statistic-wise.

Scenario (b) will require pte markers as part of the function to trap writes to
uffd-wp protected regions when the pages were e.g. swapped out or zapped for
any reason.

Currently, uffd-wp shmem work (still during review on the list, latest v5, [1])
used another solution called "special swap pte". It works similarly like PTE
markers as both of the approachs are to persist information into zapped pte,
but people showed concern about that idea and it's suggested to use a safer
(swp-entry level operation, not pte level), and arch-independent approach.

Hopefully PTE markers satifsfy these demands.

Before I rework the uffd-wp series, I wanted to know whether this approach can
be accepted upstream. So besides the swap part, comments on PTE markers will
be extremely welcomed.

For uffd-wp in its current form, it would certainly be the way to go I think. AFAIKT, the idea of special swap entries isn't new, just that it's limited to anonymous memory for now, which makes things like fork and new mappings a lot cheaper.

What is PTE Markers?

PTE markers are defined as some special PTEs that works like a "marker" just
like in normal life. Firstly it uses a swap type, IOW it's not a valid/present
pte, so processor will trigger a page fault when it's accessed. Meanwhile, the
format of the PTE is well-defined, so as to contain some information that we
would like to know before/during the page access happening.

In this specific case, when the shmem page is paged out, we set a marker
showing that this page was paged out, then when pagemap is read about this pte,
we know this is a swapped-out/very-cold page.

This use case is not an obvious one but the most simplest. The uffd-wp use
case is more obvious (wr-protect is per-pte, so we can't save into page cache;
meanwhile we need that info to persist across zappings e.g. thp split or page
out of shmem pages).

So in the future, it can contain more information, e.g., whether this pte is
wr-protected by userfaultfd; whether this pte was written in this mm context
for soft-dirtying. On 64 bit systems, we have a total of 58 bits (swp_offset).

I'm also curious whether it can be further expanded to other mm areas. E.g.,
logically it can work too for non-RAM based memories outside shmem/hugetlbfs,
e.g. a common file system like ext4 or btrfs? As long as there will be a need
to store some per-pte information across zapping of the ptes, then maybe it can
be considered.

As already expressed, we should try storing as little information in page tables as possible if we're dealing with shared memory. The features we design around this all seem to over-complicate the actual users, over-complicate fork, over-complicate handling on new mappings.

For uffd-wp in its current form, there seems to be no way around it, and PTE markers seem to be what you want -- but as I already raised, the feature itself on shmem is somewhat suboptimal, just like SOFTDIRTY tracking on shmem.

But I guess I'm biased at this point because the main users of these features actually want to query/set such properties for all sharers, not individual processes; so the opinion of others would be appreciated.

Known Issues/Concerns

About THP

Currently we don't need to worry about THP because paged out shmem pages will
be split when shrinking, IOW we only need to consider PTE, and the markers will
only be applied to a shmem pte not pmd or bigger.

About PM_SWAP Accuracy

This is not an "accurate" solution to provide PM_SWAP bit. Two exmaples:

- When process A & B both map shmem page P somewhere, it can happen that only
one of these ptes got marked with the pte marker. Imagine below sequence:

0. Process A & B both map shmem page P somewhere
1. Process A zap pte of page P for some reason (e.g. thp split)
2. System decides to recycle page P
3. System replace process B's pte (pointed to P) by PTE marker
4. System _didn't_ replace process A's pte because it was none pte, and
it'll continue to be none pte
5. Only process B's relevant pte has the PTE marker after P swapped out

- When fork, we don't copy shmem vma ptes, including the pte markers. So
even if page P was swapped out, only the parent process has the pte marker
installed, in child it'll be none pte if fork() happened after pageout.

Conclusion: just like it used to be, the PM_SWAP is best-effort. But it should
work in 99.99% cases and it should already start to solve problems.

At least I don't like these semantics at all. PM_SWAP is a cached value which might be under-represented and consequently wrong. Take CRIU as an example, it has to be correct even if a process would remap a memory region, fork() and unmap in the parent as far as I understand, ...

If we really care about performance for users with the old semantics, introduce some runtime toggle that enables the new behavior (even for a single process?) and consequently is a bit slower in corner cases. But I really do wonder if we care at all about the performance degradation in corner cases.

If we really care about performance for users with new semantics, then let's do it properly and see how we can actually speed it up without per-process page table hacks.

Anyhow, just my 2 cents.


David / dhildenb