Re: [RFC PATCH 2/2] mm: implement page refcount locking via dedicated bit

[Kiryl Shutsemau]

On Fri, Dec 19, 2025 at 10:08:54PM +0300, Gladyshev Ilya wrote:
> On 12/19/2025 8:46 PM, Kiryl Shutsemau wrote:
> > On Fri, Dec 19, 2025 at 07:18:53PM +0300, Gladyshev Ilya wrote:
> > > On 12/19/2025 5:50 PM, Kiryl Shutsemau wrote:
> > > > On Fri, Dec 19, 2025 at 12:46:39PM +0000, Gladyshev Ilya wrote:
> > > > > The current atomic-based page refcount implementation treats zero
> > > > > counter as dead and requires a compare-and-swap loop in folio_try_get()
> > > > > to prevent incrementing a dead refcount. This CAS loop acts as a
> > > > > serialization point and can become a significant bottleneck during
> > > > > high-frequency file read operations.
> > > > > 
> > > > > This patch introduces FOLIO_LOCKED_BIT to distinguish between a
> > > > 
> > > > s/FOLIO_LOCKED_BIT/PAGEREF_LOCKED_BIT/
> > > Ack, thanks
> > > 
> > > > > (temporary) zero refcount and a locked (dead/frozen) state. Because now
> > > > > incrementing counter doesn't affect it's locked/unlocked state, it is
> > > > > possible to use an optimistic atomic_fetch_add() in
> > > > > page_ref_add_unless_zero() that operates independently of the locked bit.
> > > > > The locked state is handled after the increment attempt, eliminating the
> > > > > need for the CAS loop.
> > > > 
> > > > I don't think I follow.
> > > > 
> > > > Your trick with the PAGEREF_LOCKED_BIT helps with serialization against
> > > > page_ref_freeze(), but I don't think it does anything to serialize
> > > > against freeing the page under you.
> > > > 
> > > > Like, if the page in the process of freeing, page allocator sets its
> > > > refcount to zero and your version of page_ref_add_unless_zero()
> > > > successfully acquirees reference for the freed page.
> > > > 
> > > > How is it safe?
> > > 
> > > Page is freed only after a successful page_ref_dec_and_test() call, which
> > > will set LOCKED_BIT. This bit will persist until set_page_count(1) is called
> > > somewhere in the allocation path [alloc_pages()], and effectively block any
> > > "use after free" users.
> > 
> > Okay, fair enough.
> > 
> > But what prevent the following scenario?
> > 
> > 		CPU0					CPU1
> > page_ref_dec_and_test()
> >    atomic_dec_and_test() // refcount=0
> > 					page_ref_add_unless_zero()
> > 					  atomic_add_return() // refcount=1, no LOCKED_BIT
> > 					page_ref_dec_and_test()
> > 					  atomic_dec_and_test() // refcount=0
> > 					  atomic_cmpxchg(0, LOCKED_BIT) // succeeds
> >    atomic_cmpxchg(0, LOCKED_BIT) // fails
> >    // return false to caller
> > // Use-after-free: BOOM!
> > 
> But you can't trust that the page is safe to use after
> page_ref_dec_and_test() returns false, if I understood your example
> correctly.

True. My bad.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov