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Fri, 04 Sep 2026 04:05:22 -0700 (PDT) From: Bo Zhang To: aliceryhl@google.com, gregkh@linuxfoundation.org, cmllamas@google.com Cc: arve@android.com, tkjos@android.com, christian@brauner.io, surenb@google.com, baohua@kernel.org, zhanghongru06@gmail.com, linux-kernel@vger.kernel.org, Bo Zhang Subject: [RFC PATCH v3 1/2] binder: switch alloc->mutex to spinlock for buffer metadata Date: Fri, 4 Sep 2026 19:04:47 +0800 Message-Id: <20260904110448.23086-2-zhangbo0325@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260904110448.23086-1-zhangbo0325@gmail.com> References: <20260904110448.23086-1-zhangbo0325@gmail.com> 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" From: Bo Zhang The alloc->mutex is a highly contended lock on Android devices. When a low-priority task holds this mutex and sleeps, high-priority binder transactions are blocked, causing priority inversion and latency spikes. Split the lock by converting alloc->mutex to a spinlock that only protects buffer metadata. This eliminates the sleeping and priority inversion on the hot path. Page installation and shrinker zap serialization is handled separately by a dedicated install_mutex introduced in the next patch. Performance (binderThroughputTest, SM8850, 2 workers, 10 runs): mutex spinlock throughput: 27k-59k iter/s 79k-84k iter/s average: 0.031-0.068ms 0.022-0.023ms P99: 0.088-0.148ms 0.050-0.062ms Signed-off-by: Bo Zhang --- drivers/android/binder_alloc.c | 36 +++++++++++++++++----------------- drivers/android/binder_alloc.h | 8 ++++---- 2 files changed, 22 insertions(+), 22 deletions(-) diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c index e4488ad86a65..9775df3616aa 100644 --- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -169,7 +169,7 @@ static struct binder_buffer *binder_alloc_prepare_to_fr= ee_locked( struct binder_buffer *binder_alloc_prepare_to_free(struct binder_alloc *al= loc, unsigned long user_ptr) { - guard(mutex)(&alloc->mutex); + guard(spinlock)(&alloc->lock); return binder_alloc_prepare_to_free_locked(alloc, user_ptr); } =20 @@ -676,10 +676,10 @@ struct binder_buffer *binder_alloc_new_buf(struct bin= der_alloc *alloc, if (!next) return ERR_PTR(-ENOMEM); =20 - mutex_lock(&alloc->mutex); + spin_lock(&alloc->lock); buffer =3D binder_alloc_new_buf_locked(alloc, next, size, is_async); if (IS_ERR(buffer)) { - mutex_unlock(&alloc->mutex); + spin_unlock(&alloc->lock); goto out; } =20 @@ -687,7 +687,7 @@ struct binder_buffer *binder_alloc_new_buf(struct binde= r_alloc *alloc, buffer->offsets_size =3D offsets_size; buffer->extra_buffers_size =3D extra_buffers_size; buffer->pid =3D current->tgid; - mutex_unlock(&alloc->mutex); + spin_unlock(&alloc->lock); =20 ret =3D binder_install_buffer_pages(alloc, buffer, size); if (ret) { @@ -872,9 +872,9 @@ void binder_alloc_free_buf(struct binder_alloc *alloc, binder_alloc_clear_buf(alloc, buffer); buffer->clear_on_free =3D false; } - mutex_lock(&alloc->mutex); + spin_lock(&alloc->lock); binder_free_buf_locked(alloc, buffer); - mutex_unlock(&alloc->mutex); + spin_unlock(&alloc->lock); } EXPORT_SYMBOL_IF_KUNIT(binder_alloc_free_buf); =20 @@ -967,7 +967,7 @@ void binder_alloc_deferred_release(struct binder_alloc = *alloc) struct binder_buffer *buffer; =20 buffers =3D 0; - mutex_lock(&alloc->mutex); + spin_lock(&alloc->lock); BUG_ON(alloc->mapped); =20 while ((n =3D rb_first(&alloc->allocated_buffers))) { @@ -1018,7 +1018,7 @@ void binder_alloc_deferred_release(struct binder_allo= c *alloc) page_count++; } } - mutex_unlock(&alloc->mutex); + spin_unlock(&alloc->lock); kvfree(alloc->pages); if (alloc->mm) mmdrop(alloc->mm); @@ -1043,7 +1043,7 @@ void binder_alloc_print_allocated(struct seq_file *m, struct binder_buffer *buffer; struct rb_node *n; =20 - guard(mutex)(&alloc->mutex); + guard(spinlock)(&alloc->lock); for (n =3D rb_first(&alloc->allocated_buffers); n; n =3D rb_next(n)) { buffer =3D rb_entry(n, struct binder_buffer, rb_node); seq_printf(m, " buffer %d: %lx size %zd:%zd:%zd %s\n", @@ -1069,7 +1069,7 @@ void binder_alloc_print_pages(struct seq_file *m, int lru =3D 0; int free =3D 0; =20 - mutex_lock(&alloc->mutex); + spin_lock(&alloc->lock); /* * Make sure the binder_alloc is fully initialized, otherwise we might * read inconsistent state. @@ -1085,7 +1085,7 @@ void binder_alloc_print_pages(struct seq_file *m, lru++; } } - mutex_unlock(&alloc->mutex); + spin_unlock(&alloc->lock); seq_printf(m, " pages: %d:%d:%d\n", active, lru, free); seq_printf(m, " pages high watermark: %zu\n", alloc->pages_high); } @@ -1101,7 +1101,7 @@ int binder_alloc_get_allocated_count(struct binder_al= loc *alloc) struct rb_node *n; int count =3D 0; =20 - guard(mutex)(&alloc->mutex); + guard(spinlock)(&alloc->lock); for (n =3D rb_first(&alloc->allocated_buffers); n !=3D NULL; n =3D rb_nex= t(n)) count++; return count; @@ -1161,8 +1161,8 @@ enum lru_status binder_alloc_free_page(struct list_he= ad *item, vma =3D vma_lookup(mm, page_addr); } =20 - if (!mutex_trylock(&alloc->mutex)) - goto err_get_alloc_mutex_failed; + if (!spin_trylock(&alloc->lock)) + goto err_get_alloc_lock_failed; =20 /* * Since a binder_alloc can only be mapped once, we ensure @@ -1180,6 +1180,7 @@ enum lru_status binder_alloc_free_page(struct list_he= ad *item, trace_binder_unmap_kernel_end(alloc, index); =20 list_lru_isolate(lru, item); + spin_unlock(&alloc->lock); spin_unlock(&lru->lock); =20 if (vma) { @@ -1190,7 +1191,6 @@ enum lru_status binder_alloc_free_page(struct list_he= ad *item, trace_binder_unmap_user_end(alloc, index); } =20 - mutex_unlock(&alloc->mutex); if (mm_locked) mmap_read_unlock(mm); else @@ -1201,8 +1201,8 @@ enum lru_status binder_alloc_free_page(struct list_he= ad *item, return LRU_REMOVED_RETRY; =20 err_invalid_vma: - mutex_unlock(&alloc->mutex); -err_get_alloc_mutex_failed: + spin_unlock(&alloc->lock); +err_get_alloc_lock_failed: if (mm_locked) mmap_read_unlock(mm); else @@ -1235,7 +1235,7 @@ VISIBLE_IF_KUNIT void __binder_alloc_init(struct bind= er_alloc *alloc, alloc->pid =3D current->tgid; alloc->mm =3D current->mm; mmgrab(alloc->mm); - mutex_init(&alloc->mutex); + spin_lock_init(&alloc->lock); INIT_LIST_HEAD(&alloc->buffers); alloc->freelist =3D freelist; } diff --git a/drivers/android/binder_alloc.h b/drivers/android/binder_alloc.h index d6f1f6f2d00e..bea5a77bb6da 100644 --- a/drivers/android/binder_alloc.h +++ b/drivers/android/binder_alloc.h @@ -9,7 +9,7 @@ #include #include #include -#include +#include #include #include #include @@ -80,7 +80,7 @@ static inline struct list_head *page_to_lru(struct page *= p) =20 /** * struct binder_alloc - per-binder proc state for binder allocator - * @mutex: protects binder_alloc fields + * @lock: protects binder_alloc fields * @mm: copy of task->mm (invariant after open) * @vm_start: base of per-proc address space mapped via mmap * @buffers: list of all buffers for this proc @@ -105,7 +105,7 @@ static inline struct list_head *page_to_lru(struct page= *p) * struct binder_buffer objects used to track the user buffers */ struct binder_alloc { - struct mutex mutex; 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Fri, 04 Sep 2026 04:05:28 -0700 (PDT) Received: from zhangbo56-PC.mioffice.cn ([43.224.245.235]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-39b260f64ecsm3951145a91.8.2026.09.04.04.05.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Sep 2026 04:05:28 -0700 (PDT) From: Bo Zhang To: aliceryhl@google.com, gregkh@linuxfoundation.org, cmllamas@google.com Cc: arve@android.com, tkjos@android.com, christian@brauner.io, surenb@google.com, baohua@kernel.org, zhanghongru06@gmail.com, linux-kernel@vger.kernel.org, Bo Zhang Subject: [RFC PATCH v3 2/2] binder: add install_mutex to serialize page install and shrinker zap Date: Fri, 4 Sep 2026 19:04:48 +0800 Message-Id: <20260904110448.23086-3-zhangbo0325@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260904110448.23086-1-zhangbo0325@gmail.com> References: <20260904110448.23086-1-zhangbo0325@gmail.com> 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" From: Bo Zhang The previous patch converted alloc->mutex to a spinlock for the hot path (buffer alloc/free). However, this leaves page installation and shrinker's zap_vma_range() unserialized, which can cause use-after-free as identified by Alice Ryhl. Add a separate install_mutex to serialize page installation against the shrinker's page reclaim (pages[index]=3DNULL + zap_vma_range). This mutex is only contended on the cold path when pages need to be installed or reclaimed, not on the hot path. Key changes: - binder_install_single_page() holds install_mutex across the entire install sequence, eliminating the need for binder_page_lookup() (GUP) since concurrent installers are now serialized. - The shrinker holds install_mutex across pages[index]=3DNULL and zap_vma_range(), making them atomic to the install side. - binder_lru_freelist_del() returns -EAGAIN when list_lru_del() fails (shrinker already isolated the page). Any pages already removed from the LRU are rolled back, and if the free buffer was split, the split is undone (rb_erase + list_del). The caller in binder_alloc_new_buf() reallocates the preallocated buffer on each attempt, so a freed pointer is never reused. - To avoid an ABBA deadlock (the shrinker takes mmap_lock before install_mutex, while the install side takes install_mutex first), binder_page_insert() uses mmap_read_trylock() in its fallback path and returns -EAGAIN on contention. The caller drops install_mutex and waits for mmap_lock without holding any binder lock before retrying, so the install side never blocks on mmap_lock under install_mutex. - Under install_mutex the PTE cannot already be populated, so an unexpected -EBUSY from vm_insert_page() is treated as an error rather than retried, avoiding any risk of looping. Performance (binderThroughputTest, Qualcomm SM8850, 2 workers, 10 runs) under concurrent drop_caches shows no regression from the install_mutex: mutex (baseline) spinlock + install_mutex throughput: 27k-59k iter/s 85k-89k iter/s average: 0.031-0.068ms 0.021-0.022ms P99: 0.088-0.148ms 0.046-0.056ms Signed-off-by: Bo Zhang --- drivers/android/binder_alloc.c | 108 ++++++++++++++++++++++++++------- drivers/android/binder_alloc.h | 3 + 2 files changed, 90 insertions(+), 21 deletions(-) diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c index 9775df3616aa..bbef44cc30d4 100644 --- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -268,8 +268,13 @@ static int binder_page_insert(struct binder_alloc *all= oc, return ret; } =20 - /* fall back to mmap_lock */ - mmap_read_lock(mm); + /* + * Fall back to mmap_lock. Use trylock to avoid blocking under + * install_mutex, which could deadlock against the shrinker (it + * takes mmap_lock before install_mutex). Retry on contention. + */ + if (!mmap_read_trylock(mm)) + return -EAGAIN; vma =3D vma_lookup(mm, addr); if (vma && binder_alloc_is_mapped(alloc)) ret =3D vm_insert_page(vma, addr, page); @@ -325,24 +330,36 @@ static int binder_install_single_page(struct binder_a= lloc *alloc, goto out; } =20 + mutex_lock(&alloc->install_mutex); + + /* Check again under install_mutex */ + if (binder_get_installed_page(alloc, index)) { + mutex_unlock(&alloc->install_mutex); + binder_free_page(page); + ret =3D 0; + goto out; + } + ret =3D binder_page_insert(alloc, addr, page); switch (ret) { + case -EAGAIN: + /* mmap_lock contended; drop install_mutex and retry */ + binder_free_page(page); + mutex_unlock(&alloc->install_mutex); + goto out; case -EBUSY: /* - * EBUSY is ok. Someone installed the pte first but the - * alloc->pages[index] has not been updated yet. Discard - * our page and look up the one already installed. + * install_mutex serializes page installation against the + * shrinker's zap, so the PTE should never be already + * populated here. If it somehow is (e.g. populated + * externally), fail rather than retry to avoid looping. */ - ret =3D 0; binder_free_page(page); - page =3D binder_page_lookup(alloc, addr); - if (!page) { - pr_err("%d: failed to find page at offset %lx\n", - alloc->pid, addr - alloc->vm_start); - ret =3D -ESRCH; - break; - } - fallthrough; + mutex_unlock(&alloc->install_mutex); + pr_err("%d: %s unexpected EBUSY at offset %lx\n", + alloc->pid, __func__, addr - alloc->vm_start); + ret =3D -ENOMEM; + goto out; case 0: /* Mark page installation complete and safe to use */ binder_set_installed_page(alloc, index, page); @@ -353,6 +370,8 @@ static int binder_install_single_page(struct binder_all= oc *alloc, alloc->pid, __func__, addr - alloc->vm_start, ret); break; } + + mutex_unlock(&alloc->install_mutex); out: mmput_async(alloc->mm); return ret; @@ -377,8 +396,21 @@ static int binder_install_buffer_pages(struct binder_a= lloc *alloc, continue; =20 trace_binder_alloc_page_start(alloc, index); - +retry: ret =3D binder_install_single_page(alloc, index, page_addr); + if (ret =3D=3D -EAGAIN) { + /* + * Wait for mmap_lock to become free before retrying, + * to avoid busy-looping. Safe here as no binder lock + * is held. + */ + if (mmget_not_zero(alloc->mm)) { + mmap_read_lock(alloc->mm); + mmap_read_unlock(alloc->mm); + mmput_async(alloc->mm); + } + goto retry; + } if (ret) return ret; =20 @@ -389,7 +421,7 @@ static int binder_install_buffer_pages(struct binder_al= loc *alloc, } =20 /* The range of pages should exclude those shared with other buffers */ -static void binder_lru_freelist_del(struct binder_alloc *alloc, +static int binder_lru_freelist_del(struct binder_alloc *alloc, unsigned long start, unsigned long end) { unsigned long page_addr; @@ -411,7 +443,16 @@ static void binder_lru_freelist_del(struct binder_allo= c *alloc, page_to_lru(page), page_to_nid(page), NULL); - WARN_ON(!on_lru); + /* + * If !on_lru, the shrinker has already isolated this + * page and will reclaim it. Abort so the caller can + * retry after the shrinker finishes. + */ + if (!on_lru) { + /* Rollback pages already removed from LRU */ + binder_lru_freelist_add(alloc, start, page_addr); + return -EAGAIN; + } =20 trace_binder_alloc_lru_end(alloc, index); continue; @@ -420,6 +461,8 @@ static void binder_lru_freelist_del(struct binder_alloc= *alloc, if (index + 1 > alloc->pages_high) alloc->pages_high =3D index + 1; } + + return 0; } =20 static void debug_no_space_locked(struct binder_alloc *alloc) @@ -521,6 +564,7 @@ static struct binder_buffer *binder_alloc_new_buf_locke= d( struct rb_node *n =3D alloc->free_buffers.rb_node; struct rb_node *best_fit =3D NULL; struct binder_buffer *buffer; + struct binder_buffer *split_buffer =3D NULL; unsigned long next_used_page; unsigned long curr_last_page; size_t buffer_size; @@ -568,6 +612,7 @@ static struct binder_buffer *binder_alloc_new_buf_locke= d( list_add(&new_buffer->entry, &buffer->entry); new_buffer->free =3D 1; binder_insert_free_buffer(alloc, new_buffer); + split_buffer =3D new_buffer; new_buffer =3D NULL; } =20 @@ -583,8 +628,17 @@ static struct binder_buffer *binder_alloc_new_buf_lock= ed( */ next_used_page =3D (buffer->user_data + buffer_size) & PAGE_MASK; curr_last_page =3D PAGE_ALIGN(buffer->user_data + size); - binder_lru_freelist_del(alloc, PAGE_ALIGN(buffer->user_data), - min(next_used_page, curr_last_page)); + if (binder_lru_freelist_del(alloc, PAGE_ALIGN(buffer->user_data), + min(next_used_page, curr_last_page))) { + /* Shrinker is reclaiming a page; undo the split and retry */ + if (split_buffer) { + rb_erase(&split_buffer->rb_node, &alloc->free_buffers); + list_del(&split_buffer->entry); + new_buffer =3D split_buffer; + } + buffer =3D ERR_PTR(-EAGAIN); + goto out; + } =20 rb_erase(&buffer->rb_node, &alloc->free_buffers); buffer->free =3D 0; @@ -671,7 +725,8 @@ struct binder_buffer *binder_alloc_new_buf(struct binde= r_alloc *alloc, return ERR_PTR(-EINVAL); } =20 - /* Preallocate the next buffer */ + /* Preallocate the next buffer; (re)allocate on each attempt */ +retry: next =3D kzalloc_obj(*next); if (!next) return ERR_PTR(-ENOMEM); @@ -680,6 +735,12 @@ struct binder_buffer *binder_alloc_new_buf(struct bind= er_alloc *alloc, buffer =3D binder_alloc_new_buf_locked(alloc, next, size, is_async); if (IS_ERR(buffer)) { spin_unlock(&alloc->lock); + if (PTR_ERR(buffer) =3D=3D -EAGAIN) { + /* wait for the shrinker to finish, then retry */ + mutex_lock(&alloc->install_mutex); + mutex_unlock(&alloc->install_mutex); + goto retry; + } goto out; } =20 @@ -1175,7 +1236,6 @@ enum lru_status binder_alloc_free_page(struct list_he= ad *item, trace_binder_unmap_kernel_start(alloc, index); =20 page_to_free =3D alloc->pages[index]; - binder_set_installed_page(alloc, index, NULL); =20 trace_binder_unmap_kernel_end(alloc, index); =20 @@ -1183,6 +1243,9 @@ enum lru_status binder_alloc_free_page(struct list_he= ad *item, spin_unlock(&alloc->lock); spin_unlock(&lru->lock); =20 + mutex_lock(&alloc->install_mutex); + binder_set_installed_page(alloc, index, NULL); + if (vma) { trace_binder_unmap_user_start(alloc, index); =20 @@ -1191,6 +1254,8 @@ enum lru_status binder_alloc_free_page(struct list_he= ad *item, trace_binder_unmap_user_end(alloc, index); } =20 + mutex_unlock(&alloc->install_mutex); + if (mm_locked) mmap_read_unlock(mm); else @@ -1236,6 +1301,7 @@ VISIBLE_IF_KUNIT void __binder_alloc_init(struct bind= er_alloc *alloc, alloc->mm =3D current->mm; mmgrab(alloc->mm); spin_lock_init(&alloc->lock); + mutex_init(&alloc->install_mutex); INIT_LIST_HEAD(&alloc->buffers); alloc->freelist =3D freelist; } diff --git a/drivers/android/binder_alloc.h b/drivers/android/binder_alloc.h index bea5a77bb6da..85817efdbef6 100644 --- a/drivers/android/binder_alloc.h +++ b/drivers/android/binder_alloc.h @@ -9,6 +9,7 @@ #include #include #include +#include #include #include #include @@ -81,6 +82,7 @@ static inline struct list_head *page_to_lru(struct page *= p) /** * struct binder_alloc - per-binder proc state for binder allocator * @lock: protects binder_alloc fields + * @install_mutex: serializes page installation and shrinker zap * @mm: copy of task->mm (invariant after open) * @vm_start: base of per-proc address space mapped via mmap * @buffers: list of all buffers for this proc @@ -106,6 +108,7 @@ static inline struct list_head *page_to_lru(struct page= *p) */ struct binder_alloc { spinlock_t lock; + struct mutex install_mutex; struct mm_struct *mm; unsigned long vm_start; struct list_head buffers; --=20 2.34.1