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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 =3D 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@k= ernel.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-=3Dwj00-nGmXEkxY=3D-=3DZ_qP6kiGUziS= FvxHJ9N-cLWry5zpA@mail.gmail.com/ > > Reviewed-by: Artem Kuzin > Co-developed-by: Ivan Gorbunov > Signed-off-by: Ivan Gorbunov > Signed-off-by: Ilya Gladyshev > Acked-by: Linus Torvalds > --- > include/linux/page-flags.h | 13 +++++++++++++ > include/linux/page_ref.h | 30 +++++++++++++++++++++++++----- > 2 files changed, 38 insertions(+), 5 deletions(-) > --=20 Best Regards, Yan, Zi