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

Huang, Ying
2024-05-30T01:13:41Z

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().

--
Best Regards,
Huang, Ying