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

Dave Hansen
2024-05-29T16:41:25Z

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.