Re: [PATCH] mm,unmap: avoid flushing TLB in batch if PTE is inaccessible

[Nadav Amit]

> On Apr 10, 2023, at 6:31 PM, Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> 
> !! External Email
> 
> Hi, Amit,
> 
> Thank you very much for review!
> 
> Nadav Amit <namit@xxxxxxxxxx> writes:
> 
>>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@xxxxxxxxx> wrote:
>>> 
>>> 0Day/LKP reported a performance regression for commit
>>> 7e12beb8ca2a ("migrate_pages: batch flushing TLB"). In the commit, the
>>> TLB flushing during page migration is batched.  So, in
>>> try_to_migrate_one(), ptep_clear_flush() is replaced with
>>> set_tlb_ubc_flush_pending().  In further investigation, it is found
>>> that the TLB flushing can be avoided in ptep_clear_flush() if the PTE
>>> is inaccessible.  In fact, we can optimize in similar way for the
>>> batched TLB flushing too to improve the performance.
>>> 
>>> So in this patch, we check pte_accessible() before
>>> set_tlb_ubc_flush_pending() in try_to_unmap/migrate_one().  Tests show
>>> that the benchmark score of the anon-cow-rand-mt test case of
>>> vm-scalability test suite can improve up to 2.1% with the patch on a
>>> Intel server machine.  The TLB flushing IPI can reduce up to 44.3%.
>> 
>> LGTM.
> 
> Thanks!
> 
>> I know it’s meaningless for x86 (but perhaps ARM would use this infra
>> too): do we need smp_mb__after_atomic() after ptep_get_and_clear() and
>> before pte_accessible()?
> 
> Why do we need the memory barrier?  IIUC, the PTL is locked, so PTE
> value will not be changed under us.  Anything else?

I was thinking about the ordering with respect to
atomic_read(&mm->tlb_flush_pending), which is not protected by the PTL.
I guess you can correctly argue that because of other control-flow
dependencies, the barrier is not necessary.

> 
>> In addition, if this goes into stable (based on the Fixes tag), consider
>> breaking it into 2 patches, when only one would be backported.
> 
> The fixed commit (7e12beb8ca2a ("migrate_pages: batch flushing TLB")) is
> merged by v6.3-rc1.  So this patch will only be backported to v6.3 and
> later.  Is it OK?

Of course. I wasn’t sure when the bug was introduced.