Re: [PATCH v6 3/3] mm: implement page refcount locking via dedicated bit

["Zi Yan" <ziy@nvidia.com>]

On Sat Sep 12, 2026 at 3:50 PM EDT, Ilya Gladyshev wrote:
> The current page refcount implementation uses a single counter value
> (zero) as dead. So, to prevent incrementing a dead refcount in
> folio_try_get(), it fundamentally requires a CAS loop.
>
> This CAS loop can act as a serialization point and can become a
> significant bottleneck during high-frequency file read operations
> [1][2].
>
> This patch reallocates the refcount value range:
>
> (1) refcount < 0 means dead refcount (uninit / frozen)
> (2) refcount = 0 allowed only as a temporary state (see below)
> (3) refcount > 0 is a regular reference count
>
> In other words, refcount is now split into "dead bit" and a 31-bit
> counter.
>
> Refcount decrement now works as follows:
> 1. Counter decrement
> 2. If it is now zero, try to put it deep inside the dead zone
>    (CAS to INT_MIN). Or you can view it as "set up frozen bit and reset
>    counter".
> 3. This CAS can fail only if someone grabbed a reference in-between --
>    that's okay, this page is their problem now.

I am not sure this works. David raised a concern on V4[1], where if a
folio lands to page_frag_free()'s free_frozen_pages(), it will not be
properly handled. "this page is their problem" only works if every
folio/page free functions call a generic folio/page free function that
checks the page type and does the proper handling.

Before your patches, only a speculative page getter can see any page and
folio_put() handles the last reference drop. So it is safe today.

Let me know if I miss anything.


[1] https://lore.kernel.org/linux-mm/086b119c-b777-46ae-b087-798ca3e44ecd@kernel.org/
>
> The size of the dead zone allows performing an optimistic increment
> inside page_ref_add_unless_frozen(), replacing the previous read + CAS
> loop with a single RMW operation. This reduces cache line bouncing and
> improves scalability, especially in NUMA scenarios.
>
> [1]: https://lore.kernel.org/all/20251017141536.577466-1-kirill@shutemov.name/
> [2]: https://lore.kernel.org/all/CAHk-=wj00-nGmXEkxY=-=Z_qP6kiGUziSFvxHJ9N-cLWry5zpA@mail.gmail.com/
>
> Reviewed-by: Artem Kuzin <artem.kuzin@huawei.com>
> Co-developed-by: Ivan Gorbunov <ivgorbunov@me.com>
> Signed-off-by: Ivan Gorbunov <ivgorbunov@me.com>
> Signed-off-by: Ilya Gladyshev <ilya.gladyshev@linux.dev>
> Acked-by: Linus Torvalds <torvalds@linuxfoundation.org>
> ---
>  include/linux/page-flags.h | 13 +++++++++++++
>  include/linux/page_ref.h   | 30 +++++++++++++++++++++++++-----
>  2 files changed, 38 insertions(+), 5 deletions(-)
>


-- 
Best Regards,
Yan, Zi