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

[Song Liu <songliubraving@fb.com>]

> 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. 

Thanks,
Song