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

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

[Huang Ying]

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%.

Link: https://lore.kernel.org/oe-lkp/202303192325.ecbaf968-yujie.liu@intel.com
Link: https://lore.kernel.org/oe-lkp/ab92aaddf1b52ede15e2c608696c36765a2602c1.camel@intel.com/
Fixes: 7e12beb8ca2a ("migrate_pages: batch flushing TLB")
Reported-by: kernel test robot <yujie.liu@intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
---
 mm/rmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 8632e02661ac..3c7c43642d7c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1582,7 +1582,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				 */
 				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
 
-				set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
+				if (pte_accessible(mm, pteval))
+					set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
 			} else {
 				pteval = ptep_clear_flush(vma, address, pvmw.pte);
 			}
@@ -1963,7 +1964,8 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 				 */
 				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
 
-				set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
+				if (pte_accessible(mm, pteval))
+					set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
 			} else {
 				pteval = ptep_clear_flush(vma, address, pvmw.pte);
 			}
-- 
2.39.2

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

[Nadav Amit]

> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@intel.com> 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.

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()?

In addition, if this goes into stable (based on the Fixes tag), consider
breaking it into 2 patches, when only one would be backported.

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

[Huang, Ying]

Hi, Amit,

Thank you very much for review!

Nadav Amit <namit@vmware.com> writes:

>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@intel.com> 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?

> 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?

Best Regards,
Huang, Ying

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@intel.com> wrote:
> 
> !! External Email
> 
> Hi, Amit,
> 
> Thank you very much for review!
> 
> Nadav Amit <namit@vmware.com> writes:
> 
>>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@intel.com> 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.

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

[Huang, Ying]

Nadav Amit <namit@vmware.com> writes:

>> On Apr 10, 2023, at 6:31 PM, Huang, Ying <ying.huang@intel.com> wrote:
>> 
>> !! External Email
>> 
>> Hi, Amit,
>> 
>> Thank you very much for review!
>> 
>> Nadav Amit <namit@vmware.com> writes:
>> 
>>>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@intel.com> 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.

For ordering between ptep_get_and_clear() and
atomic_read(&mm->tlb_flush_pending), I think PTL has provided the
necessary protection already.  The code path to write
mm->tlb_flush_pending is,

  tlb_gather_mmu
    inc_tlb_flush_pending       a)
  lock PTL
  change PTE                    b)
  unlock PTL
  tlb_finish_mmu
    dec_tlb_flush_pending       c)

While code path of try_to_unmap/migrate_one is,

  lock PTL
  read and change PTE           d)
  read mm->tlb_flush_pending    e)
  unlock PTL

Even if e) occurs before d), they cannot occur at the same time of b).
Do I miss anything?

Best Regards,
Huang, Ying

[snip]

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

[Nadav Amit]

> On Apr 11, 2023, at 6:50 PM, Huang, Ying <ying.huang@intel.com> wrote:
> 
> !! External Email
> 
> Nadav Amit <namit@vmware.com> writes:
> 
>>> On Apr 10, 2023, at 6:31 PM, Huang, Ying <ying.huang@intel.com> wrote:
>>> 
>>> !! External Email
>>> 
>>> Hi, Amit,
>>> 
>>> Thank you very much for review!
>>> 
>>> Nadav Amit <namit@vmware.com> writes:
>>> 
>>>>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@intel.com> 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.
> 
> For ordering between ptep_get_and_clear() and
> atomic_read(&mm->tlb_flush_pending), I think PTL has provided the
> necessary protection already.  The code path to write
> mm->tlb_flush_pending is,
> 
>  tlb_gather_mmu
>    inc_tlb_flush_pending       a)
>  lock PTL
>  change PTE                    b)
>  unlock PTL
>  tlb_finish_mmu
>    dec_tlb_flush_pending       c)
> 
> While code path of try_to_unmap/migrate_one is,
> 
>  lock PTL
>  read and change PTE           d)
>  read mm->tlb_flush_pending    e)
>  unlock PTL
> 
> Even if e) occurs before d), they cannot occur at the same time of b).
> Do I miss anything?

You didn’t miss anything. I went over the comment on
inc_tlb_flush_pending() and you follow the scheme.

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

[Huang, Ying]

Nadav Amit <namit@vmware.com> writes:

>> On Apr 11, 2023, at 6:50 PM, Huang, Ying <ying.huang@intel.com> wrote:
>> 
>> !! External Email
>> 
>> Nadav Amit <namit@vmware.com> writes:
>> 
>>>> On Apr 10, 2023, at 6:31 PM, Huang, Ying <ying.huang@intel.com> wrote:
>>>> 
>>>> !! External Email
>>>> 
>>>> Hi, Amit,
>>>> 
>>>> Thank you very much for review!
>>>> 
>>>> Nadav Amit <namit@vmware.com> writes:
>>>> 
>>>>>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@intel.com> 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.
>> 
>> For ordering between ptep_get_and_clear() and
>> atomic_read(&mm->tlb_flush_pending), I think PTL has provided the
>> necessary protection already.  The code path to write
>> mm->tlb_flush_pending is,
>> 
>>  tlb_gather_mmu
>>    inc_tlb_flush_pending       a)
>>  lock PTL
>>  change PTE                    b)
>>  unlock PTL
>>  tlb_finish_mmu
>>    dec_tlb_flush_pending       c)
>> 
>> While code path of try_to_unmap/migrate_one is,
>> 
>>  lock PTL
>>  read and change PTE           d)
>>  read mm->tlb_flush_pending    e)
>>  unlock PTL
>> 
>> Even if e) occurs before d), they cannot occur at the same time of b).
>> Do I miss anything?
>
> You didn’t miss anything. I went over the comment on
> inc_tlb_flush_pending() and you follow the scheme.

Thanks!  Can I get your acked-by or reviewed-by for this patch?

Best Regards,
Huang, Ying

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

[Nadav Amit]

> On Apr 17, 2023, at 8:17 PM, Huang, Ying <ying.huang@intel.com> wrote:
> 
> !! External Email
> 
> Nadav Amit <namit@vmware.com> writes:
>> 
>> You didn’t miss anything. I went over the comment on
>> inc_tlb_flush_pending() and you follow the scheme.
> 
> Thanks!  Can I get your acked-by or reviewed-by for this patch?

Reviewed-by: Nadav Amit <namit@vmware.com>

Thanks,
Nadav

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

[haoxin]

在 2023/4/10 下午3:52, Huang Ying 写道:
> 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%.
>
> Link: https://lore.kernel.org/oe-lkp/202303192325.ecbaf968-yujie.liu@intel.com
> Link: https://lore.kernel.org/oe-lkp/ab92aaddf1b52ede15e2c608696c36765a2602c1.camel@intel.com/
> Fixes: 7e12beb8ca2a ("migrate_pages: batch flushing TLB")
> Reported-by: kernel test robot <yujie.liu@intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Nadav Amit <namit@vmware.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: David Hildenbrand <david@redhat.com>
> ---
>   mm/rmap.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 8632e02661ac..3c7c43642d7c 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1582,7 +1582,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>   				 */
>   				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>   
> -				set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
> +				if (pte_accessible(mm, pteval))
> +					set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
>   			} else {
>   				pteval = ptep_clear_flush(vma, address, pvmw.pte);
>   			}
> @@ -1963,7 +1964,8 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>   				 */
>   				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>   
> -				set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
> +				if (pte_accessible(mm, pteval))
> +					set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));

Just a advice, can you put pte_accessible() into 
set_tlb_ubc_flush_pendin(), just like ptep_clear_flush(); so that we no 
need to add  if (pte_accessible()) in per place

where call set_tlb_ubc_flush_pending();

>   			} else {
>   				pteval = ptep_clear_flush(vma, address, pvmw.pte);
>   			}

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

[Huang, Ying]

haoxin <xhao@linux.alibaba.com> writes:

> ( 2023/4/10 \vH3:52, Huang Ying S:
>> 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%.
>>
>> Link: https://lore.kernel.org/oe-lkp/202303192325.ecbaf968-yujie.liu@intel.com
>> Link: https://lore.kernel.org/oe-lkp/ab92aaddf1b52ede15e2c608696c36765a2602c1.camel@intel.com/
>> Fixes: 7e12beb8ca2a ("migrate_pages: batch flushing TLB")
>> Reported-by: kernel test robot <yujie.liu@intel.com>
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Cc: Nadav Amit <namit@vmware.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> ---
>>   mm/rmap.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 8632e02661ac..3c7c43642d7c 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1582,7 +1582,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>   				 */
>>   				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>>   -				set_tlb_ubc_flush_pending(mm,
>> pte_dirty(pteval));
>> +				if (pte_accessible(mm, pteval))
>> +					set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
>>   			} else {
>>   				pteval = ptep_clear_flush(vma, address, pvmw.pte);
>>   			}
>> @@ -1963,7 +1964,8 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>>   				 */
>>   				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>>   -				set_tlb_ubc_flush_pending(mm,
>> pte_dirty(pteval));
>> +				if (pte_accessible(mm, pteval))
>> +					set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
>
> Just a advice, can you put pte_accessible() into
> set_tlb_ubc_flush_pendin(), just like ptep_clear_flush(); so that we
> no need to add if (pte_accessible()) in per place
>
> where call set_tlb_ubc_flush_pending();

Sounds reasonable for me, will do that in the next version.  Thanks for
suggestion.

Best Regards,
Huang, Ying

>>   			} else {
>>   				pteval = ptep_clear_flush(vma, address, pvmw.pte);
>>   			}