Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

Huang, Ying
2024-05-30T08:26:09Z

Byungchul Park <byungchul@sk.com> writes:

> On Thu, May 30, 2024 at 09:11:45AM +0800, Huang, Ying wrote:
>> Byungchul Park <byungchul@sk.com> writes:
>>
>> > On Wed, May 29, 2024 at 09:41:22AM -0700, Dave Hansen wrote:
>> >> On 5/28/24 22:00, Byungchul Park wrote:
>> >> > All the code updating ptes already performs TLB flush needed in a safe
>> >> > way if it's inevitable e.g. munmap. LUF which controls when to flush in
>> >> > a higer level than arch code, just leaves stale ro tlb entries that are
>> >> > currently supposed to be in use. Could you give a scenario that you are
>> >> > concering?
>> >>
>> >> Let's go back this scenario:
>> >>
>> >> fd = open("/some/file", O_RDONLY);
>> >> ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...);
>> >> foo1 = *ptr1;
>> >>
>> >> There's a read-only PTE at 'ptr1'. Right? The page being pointed to is
>> >> eligible for LUF via the try_to_unmap() paths. In other words, the page
>> >> might be reclaimed at any time. If it is reclaimed, the PTE will be
>> >> cleared.
>> >>
>> >> Then, the user might do:
>> >>
>> >> munmap(ptr1, PAGE_SIZE);
>> >>
>> >> Which will _eventually_ wind up in the zap_pte_range() loop. But that
>> >> loop will only see pte_none(). It doesn't do _anything_ to the 'struct
>> >> mmu_gather'.
>> >>
>> >> The munmap() then lands in tlb_flush_mmu_tlbonly() where it looks at the
>> >> 'struct mmu_gather':
>> >>
>> >> if (!(tlb->freed_tables || tlb->cleared_ptes ||
>> >> tlb->cleared_pmds || tlb->cleared_puds ||
>> >> tlb->cleared_p4ds))
>> >> return;
>> >>
>> >> But since there were no cleared PTEs (or anything else) during the
>> >> unmap, this just returns and doesn't flush the TLB.
>> >>
>> >> We now have an address space with a stale TLB entry at 'ptr1' and not
>> >> even a VMA there. There's nothing to stop a new VMA from going in,
>> >> installing a *new* PTE, but getting data from the stale TLB entry that
>> >> still hasn't been flushed.
>> >
>> > Thank you for the explanation. I got you. I think I could handle the
>> > case through a new flag in vma or something indicating LUF has deferred
>> > necessary TLB flush for it during unmapping so that mmu_gather mechanism
>> > can be aware of it. Of course, the performance change should be checked
>> > again. Thoughts?
>>
>> I suggest you to start with the simple case. That is, only support page
>> reclaiming and migration. A TLB flushing can be enforced during unmap
>> with something similar as flush_tlb_batched_pending().
>
> While reading flush_tlb_batched_pending(mm), I found it already performs
> TLB flush for the target mm, if set_tlb_ubc_flush_pending(mm) has been
> hit at least once since the last flush_tlb_batched_pending(mm).
>
> Since LUF also relies on set_tlb_ubc_flush_pending(mm), it's going to
> perform TLB flush required, in flush_tlb_batched_pending(mm) during
> munmap(). So it looks safe to me with regard to munmap() already.
>
> Is there something that I'm missing?
>
> JFYI, regarding to mmap(), I have reworked on fault handler to give up
> luf when needed in a better way.

If TLB flush is always enforced during munmap(), then your solution can
only avoid TLB flushing for page reclaiming and migration, not unmap.
Or do I miss something?

--
Best Regards,
Huang, Ying