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

Byungchul Park
2024-05-30T00:50:34Z

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?

Thanks again.

Byungchul