Date:   Tue, 14 Feb 2023 17:58:45 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Chih-En Lin <shiyn.lin@...il.com>
Cc:     Pasha Tatashin <pasha.tatashin@...een.com>,
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Subject: Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table


>>>
>>> Honestly, for improving the fork(), I have an idea to skip the per-page
>>> operation without breaking the logic. However, this will introduce the
>>> complicated mechanism and may has the overhead for other features. It
>>> might not be worth it. It's hard to strike a balance between the
>>> over-complicated mechanism with (probably) better performance and data
>>> consistency with the page status. So, I would focus on the safety and
>>> stable approach at first.
>>
>> Yes, it is most probably possible, but complexity, robustness and
>> maintainability have to be considered as well.
>>
>> Thanks for implementing this approach (only deduplication without other
>> optimizations) and evaluating it accordingly. It's certainly "cleaner", such
>> that we only have to mess with unsharing and not with other
>> accounting/pinning/mapcount thingies. But it also highlights how intrusive
>> even this basic deduplication approach already is -- and that most benefits
>> of the original approach requires even more complexity on top.
>>
>> I am not quite sure if the benefit is worth the price (I am not to decide
>> and I would like to hear other options).
> 
> I'm looking at the discussion of page table sharing in 2002 [1].
> It looks like in 2002 ~ 2006, there also have some patches try to
> improve fork().
> 
> After that, I also saw one thread which is about another shared page
> table patch's benchmark. I can't find the original patch though [2].
> But, I found the probably same patch in 2005 [3], it also mentioned
> the previous benchmark discussion:
> 
> "
> For those familiar with the shared page table patch I did a couple of years
> ago, this patch does not implement copy-on-write page tables for private
> mappings.  Analysis showed the cost and complexity far outweighed any
> potential benefit.
> "

Thanks for the pointer, interesting read. And my personal opinion is 
that part of that statement still hold true :)

> 
> However, it might be different right now. For example, the implemetation
> . We have split page table lock now, so we don't have to consider the
> page_table_share_lock thing. Also, presently, we have different use
> cases (shells [2] v.s. VM cloning and fuzzing) to consider.
> 
> Nonetheless, I still think the discussion can provide some of the mind
> to us.
> 
> BTW, It seems like the 2002 patch [1] is different from the 2002 [2]
> and 2005 [3].
> 
> [1] https://lkml.iu.edu/hypermail/linux/kernel/0202.2/0102.html
> [2] https://lore.kernel.org/linux-mm/3E02FACD.5B300794@digeo.com/
> [3] https://lore.kernel.org/linux-mm/7C49DFF721CB4E671DB260F9@%5B10.1.1.4%5D/T/#u
> 
>> My quick thoughts after skimming over the core parts of this series
>>
>> (1) forgetting to break COW on a PTE in some pgtable walker feels quite
>>      likely (meaning that it might be fairly error-prone) and forgetting
>>      to break COW on a PTE table, accidentally modifying the shared
>>      table.
> 
> Maybe I should also handle arch/ and others parts.
> I will keep looking at where I missed.

One could add sanity checks when modifying a PTE while the PTE table is 
still marked shared ... but I guess there are some valid reasons where 
we might want to modify shared PTE tables (rmap).

> 
>> (2) break_cow_pte() can fail, which means that we can fail some
>>      operations (possibly silently halfway through) now. For example,
>>      looking at your change_pte_range() change, I suspect it's wrong.
> 
> Maybe I should add WARN_ON() and skip the failed COW PTE.

One way or the other we'll have to handle it. WARN_ON() sounds wrong for 
handling OOM situations (e.g., if only that cgroup is OOM).

> 
>> (3) handle_cow_pte_fault() looks quite complicated and needs quite some
>>      double-checking: we temporarily clear the PMD, to reset it
>>      afterwards. I am not sure if that is correct. For example, what
>>      stops another page fault stumbling over that pmd_none() and
>>      allocating an empty page table? Maybe there are some locking details
>>      missing or they are very subtle such that we better document them. I
>>     recall that THP played quite some tricks to make such cases work ...
> 
> I think that holding mmap_write_lock may be enough (I added
> mmap_assert_write_locked() in the fault function btw). But, I might
> be wrong. I will look at the THP stuff to see how they work. Thanks.
> 

Ehm, but page faults don't hold the mmap lock writable? And so are other 
callers, like MADV_DONTNEED or MADV_FREE.

handle_pte_fault()->handle_pte_fault()->mmap_assert_write_locked() 
should bail out.

Either I am missing something or you didn't test with lockdep enabled :)

Note that there are upstream efforts to use only a VMA lock (and some 
people even want to perform some page faults only protected by RCU).

-- 
Thanks,

David / dhildenb

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