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

[Byungchul Park]

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