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Sun, 02 Mar 2025 18:25:37 -0800 (PST) Received: from localhost ([2401:fa00:8f:203:1513:4f61:a4d3:b418]) by smtp.gmail.com with UTF8SMTPSA id d2e1a72fcca58-7356173aedfsm5377318b3a.95.2025.03.02.18.25.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 02 Mar 2025 18:25:37 -0800 (PST) From: Sergey Senozhatsky To: Andrew Morton Cc: Yosry Ahmed , Hillf Danton , Kairui Song , Sebastian Andrzej Siewior , Minchan Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sergey Senozhatsky Subject: [PATCH v10 13/19] zsmalloc: sleepable zspage reader-lock Date: Mon, 3 Mar 2025 11:03:22 +0900 Message-ID: <20250303022425.285971-14-senozhatsky@chromium.org> X-Mailer: git-send-email 2.48.1.711.g2feabab25a-goog In-Reply-To: <20250303022425.285971-1-senozhatsky@chromium.org> References: <20250303022425.285971-1-senozhatsky@chromium.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" In order to implement preemptible object mapping we need a zspage lock that satisfies several preconditions: - it should be reader-write type of a lock - it should be possible to hold it from any context, but also being preemptible if the context allows it - we never sleep while acquiring but can sleep while holding in read mode An rwsemaphore doesn't suffice, due to atomicity requirements, rwlock doesn't satisfy due to reader-preemptability requirement. It's also worth to mention, that per-zspage rwsem is a little too memory heavy (we can easily have double digits megabytes used only on rwsemaphores). Switch over from rwlock_t to a atomic_t-based implementation of a reader-writer semaphore that satisfies all of the preconditions. The spin-lock based zspage_lock is suggested by Hillf Danton. Suggested-by: Hillf Danton Signed-off-by: Sergey Senozhatsky --- mm/zsmalloc.c | 166 ++++++++++++++++++++++++++++++++++---------------- 1 file changed, 114 insertions(+), 52 deletions(-) diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index 1424ee73cbb5..afbd72363731 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -257,6 +257,15 @@ static inline void free_zpdesc(struct zpdesc *zpdesc) __free_page(page); } =20 +#define ZS_PAGE_UNLOCKED 0 +#define ZS_PAGE_WRLOCKED -1 + +struct zspage_lock { + spinlock_t lock; + int cnt; + struct lockdep_map dep_map; +}; + struct zspage { struct { unsigned int huge:HUGE_BITS; @@ -269,7 +278,7 @@ struct zspage { struct zpdesc *first_zpdesc; struct list_head list; /* fullness list */ struct zs_pool *pool; - rwlock_t lock; + struct zspage_lock zsl; }; =20 struct mapping_area { @@ -279,6 +288,84 @@ struct mapping_area { enum zs_mapmode vm_mm; /* mapping mode */ }; =20 +static void zspage_lock_init(struct zspage *zspage) +{ + static struct lock_class_key __key; + struct zspage_lock *zsl =3D &zspage->zsl; + + lockdep_init_map(&zsl->dep_map, "zspage->lock", &__key, 0); + spin_lock_init(&zsl->lock); + zsl->cnt =3D ZS_PAGE_UNLOCKED; +} + +/* + * The zspage lock can be held from atomic contexts, but it needs to remain + * preemptible when held for reading because it remains held outside of th= ose + * atomic contexts, otherwise we unnecessarily lose preemptibility. + * + * To achieve this, the following rules are enforced on readers and writer= s: + * + * - Writers are blocked by both writers and readers, while readers are on= ly + * blocked by writers (i.e. normal rwlock semantics). + * + * - Writers are always atomic (to allow readers to spin waiting for them). + * + * - Writers always use trylock (as the lock may be held be sleeping reade= rs). + * + * - Readers may spin on the lock (as they can only wait for atomic writer= s). + * + * - Readers may sleep while holding the lock (as writes only use trylock). + */ +static void zspage_read_lock(struct zspage *zspage) +{ + struct zspage_lock *zsl =3D &zspage->zsl; + + rwsem_acquire_read(&zsl->dep_map, 0, 0, _RET_IP_); + + spin_lock(&zsl->lock); + zsl->cnt++; + spin_unlock(&zsl->lock); + + lock_acquired(&zsl->dep_map, _RET_IP_); +} + +static void zspage_read_unlock(struct zspage *zspage) +{ + struct zspage_lock *zsl =3D &zspage->zsl; + + rwsem_release(&zsl->dep_map, _RET_IP_); + + spin_lock(&zsl->lock); + zsl->cnt--; + spin_unlock(&zsl->lock); +} + +static __must_check bool zspage_write_trylock(struct zspage *zspage) +{ + struct zspage_lock *zsl =3D &zspage->zsl; + + spin_lock(&zsl->lock); + if (zsl->cnt =3D=3D ZS_PAGE_UNLOCKED) { + zsl->cnt =3D ZS_PAGE_WRLOCKED; + rwsem_acquire(&zsl->dep_map, 0, 1, _RET_IP_); + lock_acquired(&zsl->dep_map, _RET_IP_); + return true; + } + + spin_unlock(&zsl->lock); + return false; +} + +static void zspage_write_unlock(struct zspage *zspage) +{ + struct zspage_lock *zsl =3D &zspage->zsl; + + rwsem_release(&zsl->dep_map, _RET_IP_); + + zsl->cnt =3D ZS_PAGE_UNLOCKED; + spin_unlock(&zsl->lock); +} + /* huge object: pages_per_zspage =3D=3D 1 && maxobj_per_zspage =3D=3D 1 */ static void SetZsHugePage(struct zspage *zspage) { @@ -290,12 +377,6 @@ static bool ZsHugePage(struct zspage *zspage) return zspage->huge; } =20 -static void migrate_lock_init(struct zspage *zspage); -static void migrate_read_lock(struct zspage *zspage); -static void migrate_read_unlock(struct zspage *zspage); -static void migrate_write_lock(struct zspage *zspage); -static void migrate_write_unlock(struct zspage *zspage); - #ifdef CONFIG_COMPACTION static void kick_deferred_free(struct zs_pool *pool); static void init_deferred_free(struct zs_pool *pool); @@ -992,7 +1073,9 @@ static struct zspage *alloc_zspage(struct zs_pool *poo= l, return NULL; =20 zspage->magic =3D ZSPAGE_MAGIC; - migrate_lock_init(zspage); + zspage->pool =3D pool; + zspage->class =3D class->index; + zspage_lock_init(zspage); =20 for (i =3D 0; i < class->pages_per_zspage; i++) { struct zpdesc *zpdesc; @@ -1015,8 +1098,6 @@ static struct zspage *alloc_zspage(struct zs_pool *po= ol, =20 create_page_chain(class, zspage, zpdescs); init_zspage(class, zspage); - zspage->pool =3D pool; - zspage->class =3D class->index; =20 return zspage; } @@ -1217,7 +1298,7 @@ void *zs_map_object(struct zs_pool *pool, unsigned lo= ng handle, * zs_unmap_object API so delegate the locking from class to zspage * which is smaller granularity. */ - migrate_read_lock(zspage); + zspage_read_lock(zspage); read_unlock(&pool->lock); =20 class =3D zspage_class(pool, zspage); @@ -1277,7 +1358,7 @@ void zs_unmap_object(struct zs_pool *pool, unsigned l= ong handle) } local_unlock(&zs_map_area.lock); =20 - migrate_read_unlock(zspage); + zspage_read_unlock(zspage); } EXPORT_SYMBOL_GPL(zs_unmap_object); =20 @@ -1671,18 +1752,18 @@ static void lock_zspage(struct zspage *zspage) /* * Pages we haven't locked yet can be migrated off the list while we're * trying to lock them, so we need to be careful and only attempt to - * lock each page under migrate_read_lock(). Otherwise, the page we lock + * lock each page under zspage_read_lock(). Otherwise, the page we lock * may no longer belong to the zspage. This means that we may wait for * the wrong page to unlock, so we must take a reference to the page - * prior to waiting for it to unlock outside migrate_read_lock(). + * prior to waiting for it to unlock outside zspage_read_lock(). */ while (1) { - migrate_read_lock(zspage); + zspage_read_lock(zspage); zpdesc =3D get_first_zpdesc(zspage); if (zpdesc_trylock(zpdesc)) break; zpdesc_get(zpdesc); - migrate_read_unlock(zspage); + zspage_read_unlock(zspage); zpdesc_wait_locked(zpdesc); zpdesc_put(zpdesc); } @@ -1693,41 +1774,16 @@ static void lock_zspage(struct zspage *zspage) curr_zpdesc =3D zpdesc; } else { zpdesc_get(zpdesc); - migrate_read_unlock(zspage); + zspage_read_unlock(zspage); zpdesc_wait_locked(zpdesc); zpdesc_put(zpdesc); - migrate_read_lock(zspage); + zspage_read_lock(zspage); } } - migrate_read_unlock(zspage); + zspage_read_unlock(zspage); } #endif /* CONFIG_COMPACTION */ =20 -static void migrate_lock_init(struct zspage *zspage) -{ - rwlock_init(&zspage->lock); -} - -static void migrate_read_lock(struct zspage *zspage) __acquires(&zspage->l= ock) -{ - read_lock(&zspage->lock); -} - -static void migrate_read_unlock(struct zspage *zspage) __releases(&zspage-= >lock) -{ - read_unlock(&zspage->lock); -} - -static void migrate_write_lock(struct zspage *zspage) -{ - write_lock(&zspage->lock); -} - -static void migrate_write_unlock(struct zspage *zspage) -{ - write_unlock(&zspage->lock); -} - #ifdef CONFIG_COMPACTION =20 static const struct movable_operations zsmalloc_mops; @@ -1785,9 +1841,6 @@ static int zs_page_migrate(struct page *newpage, stru= ct page *page, =20 VM_BUG_ON_PAGE(!zpdesc_is_isolated(zpdesc), zpdesc_page(zpdesc)); =20 - /* We're committed, tell the world that this is a Zsmalloc page. */ - __zpdesc_set_zsmalloc(newzpdesc); - /* The page is locked, so this pointer must remain valid */ zspage =3D get_zspage(zpdesc); pool =3D zspage->pool; @@ -1803,8 +1856,15 @@ static int zs_page_migrate(struct page *newpage, str= uct page *page, * the class lock protects zpage alloc/free in the zspage. */ spin_lock(&class->lock); - /* the migrate_write_lock protects zpage access via zs_map_object */ - migrate_write_lock(zspage); + /* the zspage write_lock protects zpage access via zs_map_object */ + if (!zspage_write_trylock(zspage)) { + spin_unlock(&class->lock); + write_unlock(&pool->lock); + return -EINVAL; + } + + /* We're committed, tell the world that this is a Zsmalloc page. */ + __zpdesc_set_zsmalloc(newzpdesc); =20 offset =3D get_first_obj_offset(zpdesc); s_addr =3D kmap_local_zpdesc(zpdesc); @@ -1835,7 +1895,7 @@ static int zs_page_migrate(struct page *newpage, stru= ct page *page, */ write_unlock(&pool->lock); spin_unlock(&class->lock); - migrate_write_unlock(zspage); + zspage_write_unlock(zspage); =20 zpdesc_get(newzpdesc); if (zpdesc_zone(newzpdesc) !=3D zpdesc_zone(zpdesc)) { @@ -1971,9 +2031,11 @@ static unsigned long __zs_compact(struct zs_pool *po= ol, if (!src_zspage) break; =20 - migrate_write_lock(src_zspage); + if (!zspage_write_trylock(src_zspage)) + break; + migrate_zspage(pool, src_zspage, dst_zspage); - migrate_write_unlock(src_zspage); + zspage_write_unlock(src_zspage); =20 fg =3D putback_zspage(class, src_zspage); if (fg =3D=3D ZS_INUSE_RATIO_0) { --=20 2.48.1.711.g2feabab25a-goog