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From: "Gladyshev Ilya" Message-ID: <42f67f4ea908c862d11ca65aa3da51503bcb0962@linux.dev> TLS-Required: No Subject: [PATCH v5 2/2] mm: implement page refcount locking via dedicated bit To: "Gladyshev Ilya" Cc: "Linus Torvalds" , "Andrew Morton" , ivgorbunov@me.com, Liam.Howlett@oracle.com, apopple@nvidia.com, artem.kuzin@huawei.com, baolin.wang@linux.alibaba.com, foxido@foxido.dev, harry.yoo@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, muchun.song@linux.dev, rppt@kernel.org, surenb@google.com, vbabka@suse.cz, yuzhao@google.com, ziy@nvidia.com, pfalcato@suse.de, kirill@shutemov.name In-Reply-To: References: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 78DCE20002 X-Stat-Signature: b8r5h9os3w9y71n444fuzfnry4wepm45 X-HE-Tag: 1782499623-269827 X-HE-Meta: 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 kjwrOVr2 lR/99QRISdaGKxxRwTec0VTl3wl7XoJv8mtP2YJ0SSbfJyKOkLRYzSL1aYA6Im6fkwecwI/7mJDxb1/Q/VSK53smPPfzJ0shn7hFSF+jdXb5dd6KUnAXGEfaYy1p+JJszVX6Wt721alRU6BwYJ4tB/qBfOoreGw1FrGP7ZoEWbTo14NRoJZHL8iHusU5IhTwI37v1vNJtpQ9WzENdkLTBoYJzrO8bC6Oh3lI2DfEf8Q4NSnj2wfc/vFR/vF3d15vQRuJ/yegA2cWijTQugaF/rtJ6aVNXRD/oBMWxErvDH0HQ5KCMFP3lFUE5HT9sqTMeE6jf Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 PAGEREF_FROZEN_BIT to distinguish between a (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_add_return() 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. If locked state is detected after atomic_add(), pageref counter will be reset with CAS loop, eliminating theoretical possibility of overflow. Reviewed-by: Artem Kuzin Co-developed-by: Gorbunov Ivan Signed-off-by: Gorbunov Ivan Signed-off-by: Gladyshev Ilya Acked-by: Linus Torvalds --- include/linux/page-flags.h | 13 +++++++++++++ include/linux/page_ref.h | 32 +++++++++++++++++++++++++++----- 2 files changed, 40 insertions(+), 5 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 7223f6f4e2b4..ea9904a67334 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -196,6 +196,19 @@ enum pageflags { =20 =20#define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1) =20 +/*=20Most significant bit in page refcount */ +#define PAGEREF_FROZEN_BIT BIT(31) + +/* Page reference counter can be in 4 logical states, + * which are described below with their value representation + * state | value + * (1) safe with owners | 1...INT_MAX + * (2) safe with no owners | 0 + * (3) frozen | INT_MIN....-1 + * + * State (2) can be only temporally inside dec_and_test. + */ + #ifndef __GENERATING_BOUNDS_H =20 =20/* diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h index 61a2c9b9b73c..21257559b06f 100644 --- a/include/linux/page_ref.h +++ b/include/linux/page_ref.h @@ -64,7 +64,7 @@ static inline void __page_ref_unfreeze(struct page *pag= e, int v) =20 =20static inline bool __page_count_is_frozen(int count) { - return count =3D=3D 0; + return count & PAGEREF_FROZEN_BIT; } =20 =20static inline bool __page_is_frozen(const struct page *page) @@ -74,7 +74,12 @@ static inline bool __page_is_frozen(const struct page = *page) =20 =20static inline int page_ref_count(const struct page *page) { - return atomic_read(&page->_refcount); + int val =3D atomic_read(&page->_refcount); + + if (unlikely(val & PAGEREF_FROZEN_BIT)) + return 0; + + return val; } =20 =20/** @@ -145,7 +150,7 @@ static inline void init_page_count(struct page *page) =20 =20static inline void set_page_count_frozen(struct page *page) { - set_page_count(page, 0); + set_page_count(page, PAGEREF_FROZEN_BIT); } =20 =20static inline void page_ref_add(struct page *page, int nr) @@ -215,6 +220,9 @@ static inline int page_ref_sub_and_test(struct page *= page, int nr) int ret =3D atomic_sub_and_test(nr, &page->_refcount); VM_WARN_ON_ONCE_PAGE(__page_count_is_frozen(ret + nr), page); =20 +=09if (ret) + ret =3D !atomic_cmpxchg_relaxed(&page->_refcount, 0, PAGEREF_FROZEN_BI= T); + if (page_ref_tracepoint_active(page_ref_mod_and_test)) __page_ref_mod_and_test(page, -nr, ret); return ret; @@ -245,6 +253,9 @@ static inline int page_ref_dec_and_test(struct page *= page) int ret =3D atomic_dec_and_test(&page->_refcount); VM_WARN_ON_ONCE_PAGE(__page_count_is_frozen(ret + 1), page); =20 +=09if (ret) + ret =3D !atomic_cmpxchg_relaxed(&page->_refcount, 0, PAGEREF_FROZEN_BI= T); + if (page_ref_tracepoint_active(page_ref_mod_and_test)) __page_ref_mod_and_test(page, -1, ret); return ret; @@ -270,9 +281,20 @@ static inline int folio_ref_dec_return(struct folio = *folio) return page_ref_dec_return(&folio->page); } =20 +#define=20_PAGEREF_FROZEN_LIMIT ((1 << 30) | PAGEREF_FROZEN_BIT) + static inline bool page_ref_add_unless_frozen(struct page *page, int nr) { - bool ret =3D atomic_add_unless(&page->_refcount, nr, 0); + bool ret =3D false; + int val =3D atomic_add_return(nr, &page->_refcount); + // See PAGEREF_FROZEN_BIT declaration in page-flags.h for details + ret =3D !(val & PAGEREF_FROZEN_BIT); + + /* Undo atomic_add() if counter is locked and scary big */ + while (unlikely((unsigned int)val >=3D _PAGEREF_FROZEN_LIMIT)) { + if (atomic_try_cmpxchg_relaxed(&page->_refcount, &val, PAGEREF_FROZEN_= BIT)) + break; + } =20 =20 if (page_ref_tracepoint_active(page_ref_mod_unless)) __page_ref_mod_unless(page, nr, ret); @@ -307,7 +329,7 @@ static inline bool folio_ref_try_add(struct folio *fo= lio, int count) =20 =20static inline int page_ref_freeze(struct page *page, int count) { - int ret =3D likely(atomic_cmpxchg(&page->_refcount, count, 0) =3D=3D co= unt); + int ret =3D likely(atomic_cmpxchg(&page->_refcount, count, PAGEREF_FROZ= EN_BIT) =3D=3D count); =20 =20 if (page_ref_tracepoint_active(page_ref_freeze)) __page_ref_freeze(page, count, ret); --=20 2.54.0