Re: [PATCH v11 07/14] mm: multi-gen LRU: exploit locality in rmap

[Barry Song <21cnbao@gmail.com>]

On Thu, Jun 9, 2022 at 3:52 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Tue, Jun 7, 2022 at 5:43 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > Given we used to have a flush for clear pte young in LRU, right now we are
> > moving to nop in almost all cases for the flush unless the address becomes
> > young exactly after look_around and before ptep_clear_flush_young_notify.
> > It means we are actually dropping flush. So the question is,  were we
> > overcautious? we actually don't need the flush at all even without mglru?
>
> We stopped flushing the TLB on A bit clears on x86 back in 2014.
>
> See commit b13b1d2d8692 ("x86/mm: In the PTE swapout page reclaim case
> clear the accessed bit instead of flushing the TLB").

This is true for x86, RISC-V, powerpc and S390. but it is not true for
most platforms.

There was an attempt to do the same thing in arm64:
https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1793830.html
but arm64 still sent a nosync tlbi and depent on a deferred to dsb :
https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1794484.html

Plus, generic code will also send a tlb flush:
int ptep_clear_flush_young(struct vm_area_struct *vma,
                           unsigned long address, pte_t *ptep)
{
        int young;
        young = ptep_test_and_clear_young(vma, address, ptep);
        if (young)
                flush_tlb_page(vma, address);
        return young;
}

We used to use ptep_test_and_clear_young() only in rmap.c for page_referenced()
in 2.6.0:
https://git.kernel.org/pub/scm/linux/kernel/git/mpe/linux-fullhistory.git/tree/mm/rmap.c?h=v2.6.0
int page_referenced(struct page * page)
{
      ...
      if (ptep_test_and_clear_young(p))
      ...
}

but in 2.6.12, it has been already ptep_clear_flush_young() in
page_referenced_one()
https://git.kernel.org/pub/scm/linux/kernel/git/mpe/linux-fullhistory.git/tree/mm/rmap.c?h=v2.6.12

I failed to find the history to figure out the motivation for 2.6.12
to use ptep_clear_flush_young()
in LRU, but I am still curious how using flush or not will affect LRU
on those platforms whose
ptep_clear_flush_young() and  ptep_test_and_clear_young() are different.

>
>                Linus

Thanks
Barry