Re: [RFC PATCH v2 9/9] mm: Introduce Copy-On-Write PTE table

[David Hildenbrand <david@redhat.com>]

On 29.09.22 20:57, Chih-En Lin wrote:
> On Thu, Sep 29, 2022 at 08:38:52PM +0200, David Hildenbrand wrote:
>> On 29.09.22 20:29, Chih-En Lin wrote:
>>> On Thu, Sep 29, 2022 at 07:24:31PM +0200, David Hildenbrand wrote:
>>>>>> IMHO, a relaxed form that focuses on only the memory consumption reduction
>>>>>> could *possibly* be accepted upstream if it's not too invasive or complex.
>>>>>> During fork(), we'd do exactly what we used to do to PTEs (increment
>>>>>> mapcount, refcount, trying to clear PageAnonExclusive, map the page R/O,
>>>>>> duplicate swap entries; all while holding the page table lock), however,
>>>>>> sharing the prepared page table with the child process using COW after we
>>>>>> prepared it.
>>>>>>
>>>>>> Any (most once we want to *optimize* rmap handling) modification attempts
>>>>>> require breaking COW -- copying the page table for the faulting process. But
>>>>>> at that point, the PTEs are already write-protected and properly accounted
>>>>>> (refcount/mapcount/PageAnonExclusive).
>>>>>>
>>>>>> Doing it that way might not require any questionable GUP hacks and swapping,
>>>>>> MMU notifiers etc. "might just work as expected" because the accounting
>>>>>> remains unchanged" -- we simply de-duplicate the page table itself we'd have
>>>>>> after fork and any modification attempts simply replace the mapped copy.
>>>>>
>>>>> Agree.
>>>>> However for GUP hacks, if we want to do the COW to page table, we still
>>>>> need the hacks in this patch (using the COW_PTE_OWN_EXCLUSIVE flag to
>>>>> check whether the PTE table is available or not before we do the COW to
>>>>> the table). Otherwise, it will be more complicated since it might need
>>>>> to handle situations like while preparing the COW work, it just figuring
>>>>> out that it needs to duplicate the whole table and roll back (recover
>>>>> the state and copy it to new table). Hopefully, I'm not wrong here.
>>>>
>>>> The nice thing is that GUP itself *usually* doesn't modify page tables. One
>>>> corner case is follow_pfn_pte(). All other modifications should happen in
>>>> the actual fault handler that has to deal with such kind of unsharing either
>>>> way when modifying the PTE.
>>>>
>>>> If the pages are already in a COW-ed pagetable in the desired "shared" state
>>>> (e.g., PageAnonExclusive cleared on an anonymous page), R/O pinning of such
>>>> pages will just work as expected and we shouldn't be surprised by another
>>>> set of GUP+COW CVEs.
>>>>
>>>> We'd really only deduplicate the page table and not play other tricks with
>>>> the actual page table content that differ from the existing way of handling
>>>> fork().
>>>>
>>>> I don't immediately see why we need COW_PTE_OWN_EXCLUSIVE in GUP code when
>>>> not modifying the page table. I think we only need "we have to unshare this
>>>> page table now" in follow_pfn_pte() and inside the fault handling when GUP
>>>> triggers a fault.
>>>>
>>>> I hope my assumption is correct, or am I missing something?
>>>>
>>>
>>> My consideration is when we pinned the page and did the COW to make the
>>> page table be shared. It might not allow mapping the pinned page to R/O)
>>> into both processes.
>>>
>>> So, if the fork is working on the shared state, it needs to recover the
>>> table and copy to a new one since that pinned page will need to copy
>>> immediately. We can hold the shared state after occurring such a
>>> situation. So we still need some trick to let the fork() know which page
>>> table already has the pinned page (or such page won't let us share)
>>> before going to duplicate.
>>>
>>> Am I wrong here?
>>
>> I think you might be overthinking this. Let's keep it simple:
>>
>> 1) Handle pinned anon pages just as I described below, falling back to the
>> "slow" path of page table copying.
>>
>> 2) Once we passed that stage, you can be sure that the COW-ed page table
>> cannot have actually pinned anon pages. All anon pages in such a page table
>> have PageAnonExclusive cleared and are "maybe shared". GUP cannot succeed in
>> pinning these pages anymore, because it will only pin exclusive anon pages!
>>
>> 3) If anybody wants to take a R/O pin on a shared anon page that is mapped
>> into a COW-ed page table, we trigger a fault with FAULT_FLAG_UNSHARE instead
>> of pinning the page. This has to break COW on the page table and properly
>> map an exclusive anon page into it, breaking COW.
>>
>> Do you see a problem with that?
>>
>>>
>>> After that, since we handled the accounting in fork(), we don't need
>>> ownership (pmd_t pointer) anymore. We have to find another way to mark
>>> the table to be exclusive. (Right now, COW_PTE_OWNER_EXCLUSIVE flag is
>>> stored at that space.)
>>>
>>>>>
>>>>>> But devil is in the detail (page table lock, TLB flushing).
>>>>>
>>>>> Sure, it might be an overhead in the page fault and needs to be handled
>>>>> carefully. ;)
>>>>>
>>>>>> "will make fork() even have more overhead" is not a good excuse for such
>>>>>> complexity/hacks -- sure, it will make your benchmark results look better in
>>>>>> comparison ;)
>>>>>
>>>>> ;);)
>>>>> I think that, even if we do the accounting with the COW page table, it
>>>>> still has a little bit improve.
>>>>
>>>> :)
>>>>
>>>> My gut feeling is that this is true. While we have to do a pass over the
>>>> parent page table during fork and wrprotect all PTEs etc., we don't have to
>>>> duplicate the page table content and allocate/free memory for that.
>>>>
>>>> One interesting case is when we cannot share an anon page with the child
>>>> process because it maybe pinned -- and we have to copy it via
>>>> copy_present_page(). In that case, the page table between the parent and the
>>>> child would differ and we'd not be able to share the page table.
>>>
>>> That is what I want to say above.
>>> The case might happen in the middle of the shared page table progress.
>>> It might cost more overhead to recover it. Therefore, if GUP wants to
>>> pin the mapped page we can mark the PTE table first, so fork() won't
>>> waste time doing the work for sharing.
>>
>> Having pinned pages is a corner case for most apps. No need to worry about
>> optimizing this corner case for now.
>>
>> I see what you are trying to optimize, but I don't think this is needed in a
>> first version, and probably never is needed.
>>
>>
>> Any attempts to mark page tables in a certain way from GUP
>> (COW_PTE_OWNER_EXCLUSIVE) is problematic either way: GUP-fast
>> (get_user_pages_fast) can race with pretty much anything, even with
>> concurrent fork. I suspect your current code might be really racy in that
>> regard.
> 
> I see.
> Now, I know why optimizing that corner case is not worth it.
> Thank you for explaining that.

Falling back after already processing some PTEs requires some care, 
though. I guess it's not too hard to get it right -- it might be harder 
to get it "clean". But we can talk about that detail later.

-- 
Thanks,

David / dhildenb