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

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