Re: [PATCH v4 0/2] mm: improve folio refcount scalability

[Ilya Gladyshev]

> 
> On Sun, Jun 21, 2026 at 09:34:47PM +0000, Gladyshev Ilya wrote:
> 
> > 
> > June 21, 2026 at 7:46 AM, Linus Torvalds wrote:
> >  
> >  On Sat, 20 Jun 2026 at 11:19, <ilya.gladyshev@xxxxxxxxx> wrote:
> >  
> >  > 
> >  > T2: optimistic get() [0 -> 1]
> >  > T2: put page back [1 -> 0]
> >  > T2: calls dtor for type X, returns into the allocator
> >  > 
> >  Which optimistic getter does this?
> >  
> >  If I understood you correctly, you are talking about the scenario where
> >  an optimistic getter took a refcount on the stolen page, so the validity
> >  check in the XArray will fail. And this scenario does indeed work normally.
> >  
> >  This "ABA" happens if the optimistic getter successfully gets a refcount
> >  on a valid page, so the full T2 execution looks like this:
> >  
> >  T2: optimistic get() [0 -> 1]
> >  T2: re-checks page [OK]
> > 
> I don't think that can happen. Or maybe it can and we need to add
> some barriers. The page is always removed from visibility (whether
> we're talking about a page cache lookup or a page table lookup), then
> the refcount is decremented. I hope we have enough barriers in place
> to ensure that the refcount decrement is observed after the removal of
> the PTE entry or the XArray entry.
> 
> But I'm not sure why the folio_put() after a speculative get avoids this
> problem; why do we need the recheck to be successful to hit this race?

After additional thought, this race doesn't require speculative gets at all
and is more about two parallel `folio_puts()`: one successfully deallocates
the page, and one sleeps for a long time and then calls __folio_put() on a
"logically new" page.

And as everybody pointed out, this race isn't something specific to this patch.
So folio_put() was always ready to be called on non-folio objects due to
optimistic try_get() + put() in the filecache. We only care about calling
folio_put() once, and nothing breaks with this patch.

So thanks to you, Linus, and David, for helping to clarify this :) I'll post
v5 with more or less cosmetic fixes from the AI review then.