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

Byungchul Park
2024-05-31T02:21:05Z

On Fri, May 31, 2024 at 09:45:33AM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@sk.com> writes:
>
> > On Thu, May 30, 2024 at 04:24:12PM +0800, Huang, Ying wrote:
> >> 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.
> >
> > I'm not sure if I understand what you meant. Could you explain it in
> > more detail?
> >
> > LUF works for only *unmapping* that happens during page reclaiming and
> > migration. Other unmappings than page reclaiming and migration are not
> > what LUF works for. That's why I thought flush_tlb_batched_pending()
> > could handle the pending tlb flushes in the case.
> >
> > It'd be appreciated if you explain what you meant more.
> >
>
> In the following email, you have claimed that LUF can avoid TLB flushing
> for munmap()/mmap().

My bad. Sorry for that confusing expression.

"give up LUF at mmap()" doesn't mean giving up applying LUF to mmap().

"give up LUF at mmap()" means giving up the pending that has been
induced by LUF, in other words, giving up the benefit by LUF because we
are going through mmap() / munmap().

I will be more careful in expressing these things.

> https://lore.kernel.org/linux-mm/20240527015732.GA61604@system.software.com/
>
> Now, you said it can only avoid TLB flushing for page reclaiming and
> migration.

This is true.

Byungchul

> So, to avoid confusion, I suggest you to send out a new series and make
> it explicit that it can only optimize page reclaiming and migration, but
> not munmap(). And it would be good too to add some words about how it
> interact with other TLB flushing mechanisms.
>
> --
> Best Regards,
> Huang, Ying