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

[Barry Song]

On Tue, Jun 7, 2022 at 10:21 PM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Tue, Jun 07, 2022 at 07:37:10PM +1200, Barry Song wrote:
> > On Mon, Jun 6, 2022 at 9:25 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > > On Wed, May 18, 2022 at 4:49 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> > > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > > index fedb82371efe..7cb7ef29088a 100644
> > > > --- a/mm/rmap.c
> > > > +++ b/mm/rmap.c
> > > > @@ -73,6 +73,7 @@
> > > >  #include <linux/page_idle.h>
> > > >  #include <linux/memremap.h>
> > > >  #include <linux/userfaultfd_k.h>
> > > > +#include <linux/mm_inline.h>
> > > >
> > > >  #include <asm/tlbflush.h>
> > > >
> > > > @@ -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?  is it
> > > safe to call ptep_test_and_clear_young() only?
> >
> > I can't really explain why we are getting a random app/java vm crash in monkey
> > test by using ptep_test_and_clear_young() only in lru_gen_look_around() on an
> > armv8-a machine without hardware PTE young support.
> >
> > Moving to  ptep_clear_flush_young() in look_around can make the random
> > hang disappear according to zhanyuan(Cc-ed).
> >
> > On x86, ptep_clear_flush_young() is exactly ptep_test_and_clear_young()
> > after
> >  'commit b13b1d2d8692 ("x86/mm: In the PTE swapout page reclaim case clear
> > the accessed bit instead of flushing the TLB")'
> >
> > But on arm64, they are different. according to Will's comments in this
> > thread which
> > tried to make arm64 same with x86,
> > https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1793881.html
> >
> > "
> > This is blindly copied from x86 and isn't true for us: we don't invalidate
> > the TLB on context switch. That means our window for keeping the stale
> > entries around is potentially much bigger and might not be a great idea.
> >
> > If we roll a TLB invalidation routine without the trailing DSB, what sort of
> > performance does that get you?
> > "
> > We shouldn't think ptep_clear_flush_young() is safe enough in LRU to
> > clear PTE young? Any comments from Will?
>
> Given that this issue is specific to the multi-gen LRU work, I think Yu is
> the best person to comment. However, looking quickly at your analysis above,
> I wonder if the code is relying on this sequence:
>
>
>         ptep_test_and_clear_young(vma, address, ptep);
>         ptep_clear_flush_young(vma, address, ptep);
>
>
> to invalidate the TLB. On arm64, that won't be the case, as the invalidation
> in ptep_clear_flush_young() is predicated on the pte being young (and this
> patches the generic implementation in mm/pgtable-generic.c. In fact, that
> second function call is always going to be a no-op unless the pte became
> young again in the middle.

Hi Will,
thanks for your reply, sorry for failing to let you understand my question.
my question is actually as below,
right now  lru_gen_look_around() is using ptep_test_and_clear_young()
only without flush to clear pte for a couple of pages including the specific
address:
void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
{
       ...

       for (i = 0, addr = start; addr != end; i++, addr += PAGE_SIZE) {
               ...

               if (!ptep_test_and_clear_young(pvmw->vma, addr, pte + i))
                       continue;

               ...
}

I wonder if it is safe to arm64. Do we need to move to ptep_clear_flush_young()
in the loop?

>
> Will

Thanks
Barry