Re: [PATCH] mm: memory: check userfaultfd_wp() in vmf_orig_pte_uffd_wp()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/4/18 9:47, Kefeng Wang wrote:
> 
> 
> On 2024/4/18 2:34, Peter Xu wrote:
>> Hi, Kefeng,
>>
>> On Wed, Apr 17, 2024 at 05:30:40PM +0800, Kefeng Wang wrote:
>>>
>>>
>>> On 2024/4/17 16:23, Kefeng Wang wrote:
>>>> Directly call vmf_orig_pte_uffd_wp() in do_anonymous_page() and
>>>> set_pte_range() to save a uffd_wp and add userfaultfd_wp() check
>>>> in vmf_orig_pte_uffd_wp() to avoid the unnecessary function calls
>>>> in the most sense, lat_pagefault testcase does show improvement
>>>> though very small(~1%).
>>
>> I'm ok with the change if that helps as big as 1%, but I'm a bit 
>> surprised
>> to see such a difference, because for file pte_marker_uffd_wp() should
>> check first on pte_none() then it should return already if uffd not even
>> registered for the vma, while orig_pte should be hot too if valid.
> 
> Yes, retest, not as big as 1%, but the perf shows vmf_orig_pte_uffd_wp
> is eliminated,
> 
> [root@localhost]# perf report -i perf.data.old |grep vmf
>       0.17%     0.13%  lat_pagefault  [kernel.kallsyms]      [k] 
> vmf_orig_pte_uffd_wp.part.0.isra.0
> [root@localhost]# perf report -i perf.data |grep vmf
> 
>>
>>>>
>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>> ---
>>>>    mm/memory.c | 9 +++++----
>>>>    1 file changed, 5 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 5ae2409d3cb9..a6afc96001e6 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -117,6 +117,9 @@ static bool vmf_orig_pte_uffd_wp(struct vm_fault 
>>>> *vmf)
>>>
>>>
>>>      if (!IS_ENABLED(CONFIG_PTE_MARKER_UFFD_WP))
>>>              return false;
>>>
>>> Will add config check too,
>>
>> pte_marker_uffd_wp() returns false when !PTE_MARKER_UFFD_WP, so kind of
>> imply this.  I assume you meant to avoid checking ORIG_PTE_VALID flag, 
>> but
> 
> Just to avoid checking ORIG_PTE_VALID and the new userfaultfd_wp() since
> it is not supported on most archs.

Since pte_mkuffd_wp(return pte) if !PTE_MARKER_UFFD_WP, so this is not
needed, also confirm it after checking the disassemble, will update
changelog and resend.


> 
>> the flags is pretty hot too.  Again, just want to double check with 
>> you on
>> whether it can have such a huge difference, e.g., how that compares with
>> the current code v.s. original patch v.s. this squashed.
> 
> I will change the changelog to show different about vmf_orig_pte_uffd_wp 
>   from perf data.
> 
> Thanks.
>