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.
