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

[Huang, Ying]

Byungchul Park <byungchul@xxxxxx> 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