Re: [PATCH 0/3] mm,thp,rmap: rework the use of subpages_mapcount

[Shakeel Butt <shakeelb@google.com>]

On Mon, Nov 21, 2022 at 09:16:58AM -0800, Linus Torvalds wrote:
> On Mon, Nov 21, 2022 at 8:59 AM Shakeel Butt <shakeelb@google.com> wrote:
> >
> > Is there a plan to remove lock_page_memcg() altogether which I missed? I
> > am planning to make lock_page_memcg() a nop for cgroup-v2 (as it shows
> > up in the perf profile on exit path)
> 
> Yay. It seems I'm not the only one hating it.
> 
> > but if we are removing it then I should just wait.
> 
> Well, I think Johannes was saying that at least the case I disliked
> (the rmap removal from the page table tear-down - I strongly suspect
> it's the one you're seeing on your perf profile too)

Yes indeed that is the one.

-   99.89%     0.00%  fork-large-mmap  [kernel.kallsyms]  [k] entry_SYSCALL_64_after_hw◆
     entry_SYSCALL_64_after_hwframe                               
   - do_syscall_64                                                
      - 48.94% __x64_sys_exit_group                               
           do_group_exit                                          
         - do_exit                                                
            - 48.94% exit_mm                                      
                 mmput                                            
               - __mmput                                          
                  - exit_mmap                                     
                     - 48.61% unmap_vmas                          
                        - 48.61% unmap_single_vma                 
                           - unmap_page_range                     
                              - 48.60% zap_p4d_range              
                                 - 44.66% zap_pte_range           
                                    + 12.61% tlb_flush_mmu        
                                    - 9.38% page_remove_rmap      
                                         2.50% lock_page_memcg    
                                         2.37% unlock_page_memcg  
                                         0.61% PageHuge           
                                      4.80% vm_normal_page        
                                      2.56% __tlb_remove_page_size
                                      0.85% lock_page_memcg       
                                      0.53% PageHuge              
                                   2.22% __tlb_remove_page_size   
                                   0.93% vm_normal_page           
                                   0.72% page_remove_rmap

> can be removed
> entirely as long as it's done under the page table lock (which my
> final version of the rmap delaying still was).
> 
> See
> 
>     https://lore.kernel.org/all/Y2llcRiDLHc2kg%2FN@cmpxchg.org/
> 
> for his preliminary patch.
> 
> That said, if you have some patch to make it a no-op for _other_
> reasons, and could be done away with _entirely_ (not just for rmap),
> then that would be even better.

I am actually looking at deprecating the whole "move charge"
funcitonality of cgroup-v1 i.e. the underlying reason lock_page_memcg
exists. That already does not work for couple of cases like partially
mapped THP and madv_free'd pages. Though that deprecation process would
take some time. In the meantime I was looking at if we can make these
functions nop for cgroup-v2.

thanks,
Shakeel