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

[Yu Zhao]

On Mon, Jun 6, 2022 at 3:25 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Wed, May 18, 2022 at 4:49 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:

...

> > @@ -821,6 +822,12 @@ static bool folio_referenced_one(struct folio *folio,
> >                 }
> >
> >                 if (pvmw.pte) {
> > +                       if (lru_gen_enabled() && pte_young(*pvmw.pte) &&
> > +                           !(vma->vm_flags & (VM_SEQ_READ | VM_RAND_READ))) {
> > +                               lru_gen_look_around(&pvmw);
> > +                               referenced++;
> > +                       }
> > +
> >                         if (ptep_clear_flush_young_notify(vma, address,
>
> Hello, Yu.
> look_around() is calling ptep_test_and_clear_young(pvmw->vma, addr, pte + i)
> only without flush and notify. for flush, there is a tlb operation for arm64:
> static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
>                                          unsigned long address, pte_t *ptep)
> {
>         int young = ptep_test_and_clear_young(vma, address, ptep);
>
>         if (young) {
>                 /*
>                  * We can elide the trailing DSB here since the worst that can
>                  * happen is that a CPU continues to use the young entry in its
>                  * TLB and we mistakenly reclaim the associated page. The
>                  * window for such an event is bounded by the next
>                  * context-switch, which provides a DSB to complete the TLB
>                  * invalidation.
>                  */
>                 flush_tlb_page_nosync(vma, address);
>         }
>
>         return young;
> }
>
> Does it mean the current kernel is over cautious?

Hi Barry,

This is up to individual archs. For x86, ptep_clear_flush_young() is
ptep_test_and_clear_young(). For arm64, I'd say yes, based on Figure 1
of Navarro, Juan, et al. "Practical, transparent operating system
support for superpages." [1].

int ptep_clear_flush_young(struct vm_area_struct *vma,
                           unsigned long address, pte_t *ptep)
{
        /*
         * On x86 CPUs, clearing the accessed bit without a TLB flush
         * doesn't cause data corruption. [ It could cause incorrect
         * page aging and the (mistaken) reclaim of hot pages, but the
         * chance of that should be relatively low. ]
         *
         * So as a performance optimization don't flush the TLB when
         * clearing the accessed bit, it will eventually be flushed by
         * a context switch or a VM operation anyway. [ In the rare
         * event of it not getting flushed for a long time the delay
         * shouldn't really matter because there's no real memory
         * pressure for swapout to react to. ]
         */
        return ptep_test_and_clear_young(vma, address, ptep);
}

[1] https://www.usenix.org/legacy/events/osdi02/tech/full_papers/navarro/navarro.pdf

> is it
> safe to call ptep_test_and_clear_young() only?

Yes. Though the h/w A-bit is designed to allow OSes to skip TLB
flushes when unmapping, the Linux kernel doesn't do this.

> btw, lru_gen_look_around() has already included 'address', are we doing
> pte check for 'address' twice here?

Yes for host MMU but no KVM MMU. ptep_clear_flush_young_notify() goes
into the MMU notifier. We don't use the _notify variant in
lru_gen_look_around() because GPA space generally exhibits no memory
locality.

Thanks.