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

Byungchul Park
2024-05-30T08:41:26Z

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.
^
munmap()?

Do you mean munmap()? IIUC, yes. LUF only works for page reclaiming
and migration, but not for munmap(). When munmap()ing, LUF rather needs
to give up and perform tlb flush pended.

LUF should not optimize tlb flushes for mappings that users explicitly
change e.g. through mmap() and munmap().

Byungchul

> Or do I miss something?
>
> --
> Best Regards,
> Huang, Ying