Re: [PATCH v4 5/5] uprobe: collapse THP pmd after removing all uprobes

[Song Liu <songliubraving@fb.com>]

> On Jun 24, 2019, at 5:34 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> 
> On Fri, Jun 21, 2019 at 06:04:14PM +0000, Song Liu wrote:
>> 
>> 
>>> On Jun 21, 2019, at 9:30 AM, Song Liu <songliubraving@fb.com> wrote:
>>> 
>>> 
>>> 
>>>> On Jun 21, 2019, at 6:45 AM, Song Liu <songliubraving@fb.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Jun 21, 2019, at 6:36 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>>>>> 
>>>>> On Fri, Jun 21, 2019 at 01:17:05PM +0000, Song Liu wrote:
>>>>>> 
>>>>>> 
>>>>>>> On Jun 21, 2019, at 5:48 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>>>>>>> 
>>>>>>> On Thu, Jun 13, 2019 at 10:57:47AM -0700, Song Liu wrote:
>>>>>>>> After all uprobes are removed from the huge page (with PTE pgtable), it
>>>>>>>> is possible to collapse the pmd and benefit from THP again. This patch
>>>>>>>> does the collapse.
>>>>>>>> 
>>>>>>>> An issue on earlier version was discovered by kbuild test robot.
>>>>>>>> 
>>>>>>>> Reported-by: kbuild test robot <lkp@intel.com>
>>>>>>>> Signed-off-by: Song Liu <songliubraving@fb.com>
>>>>>>>> ---
>>>>>>>> include/linux/huge_mm.h |  7 +++++
>>>>>>>> kernel/events/uprobes.c |  5 ++-
>>>>>>>> mm/huge_memory.c        | 69 +++++++++++++++++++++++++++++++++++++++++
>>>>>>> 
>>>>>>> I still sync it's duplication of khugepaged functinallity. We need to fix
>>>>>>> khugepaged to handle SCAN_PAGE_COMPOUND and probably refactor the code to
>>>>>>> be able to call for collapse of particular range if we have all locks
>>>>>>> taken (as we do in uprobe case).
>>>>>>> 
>>>>>> 
>>>>>> I see the point now. I misunderstood it for a while. 
>>>>>> 
>>>>>> If we add this to khugepaged, it will have some conflicts with my other 
>>>>>> patchset. How about we move the functionality to khugepaged after these
>>>>>> two sets get in? 
>>>>> 
>>>>> Is the last patch of the patchset essential? I think this part can be done
>>>>> a bit later in a proper way, no?
>>>> 
>>>> Technically, we need this patch to regroup pmd mapped page, and thus get 
>>>> the performance benefit after the uprobe is detached. 
>>>> 
>>>> On the other hand, if we get the first 4 patches of the this set and the 
>>>> other set in soonish. I will work on improving this patch right after that..
>>> 
>>> Actually, it might be pretty easy. We can just call try_collapse_huge_pmd() 
>>> in khugepaged.c (in khugepaged_scan_shmem() or khugepaged_scan_file() after 
>>> my other set). 
>>> 
>>> Let me fold that in and send v5. 
>> 
>> On a second thought, if we would have khugepaged to do collapse, we need a
>> dedicated bit to tell khugepaged which pmd to collapse. Otherwise, it may 
>> accidentally collapse pmd that are split by other split_huge_pmd. 
> 
> Why is it a problem? Do you know a situation where such collapse possible
> and will break split_huge_pmd() user's expectation. If there's such user
> it is broken: normal locking should prevent such situation.
> 

You are right. I found the the same after a third thought. So I tried to 
get that logic in v6. 

Thanks,
Song