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

[Kirill A. Shutemov]

On Fri, Jun 21, 2019 at 06:04:14PM +0000, Song Liu wrote:
> 
> 
> > On Jun 21, 2019, at 9:30 AM, Song Liu <songliubraving@xxxxxx> wrote:
> > 
> > 
> > 
> >> On Jun 21, 2019, at 6:45 AM, Song Liu <songliubraving@xxxxxx> wrote:
> >> 
> >> 
> >> 
> >>> On Jun 21, 2019, at 6:36 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxx>
> >>>>>> Signed-off-by: Song Liu <songliubraving@xxxxxx>
> >>>>>> ---
> >>>>>> 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.

-- 
 Kirill A. Shutemov