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

[Will Deacon]

On Tue, Jun 07, 2022 at 10:37:46AM +1200, Barry Song wrote:
> 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:
> > > 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.
> 
> 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?

I don't know what this code is doing, so Yu is the best person to answer
that. There's nothing inherently dangerous about eliding the TLB
maintenance; it really depends on the guarantees needed by the caller.

However, the snippet you posted from folio_referenced_one():

 |                  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,


Does seem to call lru_gen_look_around() *and*
ptep_clear_flush_young_notify(), which is what prompted my question as it
looks pretty suspicious to me.

Will