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

Byungchul Park
2024-05-30T01:00:04Z

On Thu, May 30, 2024 at 09:50:26AM +0900, Byungchul Park wrote:
> 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 will check the existing optimization of TLB flsuh more in arch level
and suggest a better way.

Byungchul

> Thanks again.
>
> Byungchul