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

[Yu Zhao]

On Wed, Jun 8, 2022 at 4:46 PM Barry Song <21cnbao at gmail.com> wrote:
>
> On Thu, Jun 9, 2022 at 3:52 AM Linus Torvalds
> <torvalds at linux-foundation.org> wrote:
> >
> > On Tue, Jun 7, 2022 at 5:43 PM Barry Song <21cnbao at 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

Barry, you've already answered your own question.

Without commit 07509e10dcc7 arm64: pgtable: Fix pte_accessible():
   #define pte_accessible(mm, pte)        \
  -       (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte))
  +       (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))

You missed all TLB flushes for PTEs that have gone through
ptep_test_and_clear_young() on the reclaim path. But most of the time,
you got away with it, only occasional app crashes:
https://lore.kernel.org/r/CAGsJ_4w6JjuG4rn2P=d974wBOUtXUUnaZKnx+-G6a8_mSROa+Q@mail.gmail.com/

Why?