From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f70.google.com (mail-pj1-f70.google.com [209.85.216.70]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 23FCB3B2FD0 for ; Thu, 10 Sep 2026 23:47:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.70 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789084078; cv=none; b=J4Y7UiR0jydjI9Ln9I1zzdhFxmDZh24QbJ8HC7QMPRHE4PG9u9UlpLZLX8zVjmukaQ3uYKPtSA1RY5dMVJcqFWmUbLtw9Nsjkg6sM/6YPJLWZDIcfwDLRJkR4UL36g5vNpJMEBKra4eK2RE7RimhbzyMCtMceG97hHXiGlxX2bA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789084078; c=relaxed/simple; bh=fzJ4mq+XIyRQpS9OR9S/Jsq3hBniXQdj+ylfg1W0jgk=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=WU6FOoD2ZE05i+u9NU3w5JUsD37pD9KmLD5WbrA6HM3XBgzX5Ml8FSOyqtDraNy09XuY1LNen5TdPjiOVhacSPUtRrlD7KtRLs73cPr387LTH7kIMVEBDZCeweRV4s767Kcq/qXNxxy7qia4vXuzii0BjbL7PPLUJ4PBugx93D4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--surenb.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=DBUOUZGY; arc=none smtp.client-ip=209.85.216.70 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--surenb.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="DBUOUZGY" Received: by mail-pj1-f70.google.com with SMTP id 98e67ed59e1d1-39af92138f9so600420a91.0 for ; Thu, 10 Sep 2026 16:47:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1789084076; x=1789688876; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:from:to:cc:subject:date:message-id:reply-to :content-type; bh=8UPlg4KrV740Rib8wJaNrr1K8hSvsKhdw/jziCdqQZY=; b=DBUOUZGYtEChgwqiNI4MlTq/iyI2O6RTf2Im/K3ZqjXhMbBkUvmWR7V7AG/Ye1YWMK gmzjuY2XS1xzMY5pU2HEDJB9hjxMUfE6zdUEYP5E8HC6vKZVDfjE0TJvQGMRNQgl/uZy h13fuApgJgjtkkBwFsaq0Mqgk+7DevHWw8XBZxE9lEha3otx19eFVmx93QFo4L9LvuBE 0NBnNXpC2brZ6O0IWNjePzZLpZh5hdmmj3Dw9zfjPxaTjRwVSeMiHC9wd6W8+M7wYphh zc9RkrNZovwctO2q169NHhHHGCRc2TgwPJlHAInFk35Ag28l07iYArHTtyQ69n9P3sPp LUqg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1789084076; x=1789688876; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=8UPlg4KrV740Rib8wJaNrr1K8hSvsKhdw/jziCdqQZY=; b=P+XXIfe8l7IWZ8kW7P0RsL+LqYTTgxg2MIbTdxrbXXhc7Vy41AZ5fsS03F5FG57/t6 5LQlT69S1HoHq4OFSH8Cv/0bIGzvcOiUIYNFfzy+9Hx717eaSPppIL/9p03WwCikRPeM 5jdlOM6mbvLRVUFxSFtJi7EPHVQJRYVOhv1U2mcpuIOXR09np1UvaV8dpoM5Esfg1YQr FvAIGR9Nw1FP16AgUTRF3K3p5vgfdK2HFrEU2nMoiaPVmArG4hyOXL6nANyNtSHVJK1M JztNg1Rqwl1SytWc309AYrkPh01x+AvTiPRwTfqfWGLntzrbIHODgK3gZDaXwdCIkBcO gSTw== X-Forwarded-Encrypted: i=1; AKwUvBz6z0KtERCCsvdDZDQfnrI5OtN65X+Z+atqmvDbcDyHZyVoICY2PtMHLsm9ZmF9pDOx0E33kphwDHCv+Qc=@vger.kernel.org X-Gm-Message-State: AFuF++k9YLzHOIeBJrh2ceMYd58xKBk7X+2cI28oA2IwijtGqgtojnLL S5TghKWHru+rvrut6Ht643JvQ/mk3Diekh6Liqmu0e/I6DCxhdyjRDv8cmeEJ7xkDaEO1qJ2qn8 RrxtWlA== X-Received: from dybdo17.prod.google.com ([2002:a05:7300:bc11:b0:339:79b6:13ec]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:288e:b0:39b:5622:d645 with SMTP id 98e67ed59e1d1-39d9782102dmr1952912a91.0.1789084076389; Thu, 10 Sep 2026 16:47:56 -0700 (PDT) Date: Thu, 10 Sep 2026 16:47:36 -0700 In-Reply-To: <20260910234737.1340642-1-surenb@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260910234737.1340642-1-surenb@google.com> X-Mailer: git-send-email 2.55.0.1007.g17ff1f9808-goog Message-ID: <20260910234737.1340642-7-surenb@google.com> Subject: [PATCH v3 6/7] proc/task_mmu: read proc/pid/smaps_rollup under per-vma lock From: Suren Baghdasaryan To: akpm@linux-foundation.org Cc: liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, david@redhat.com, willy@infradead.org, jannh@google.com, paulmck@kernel.org, pfalcato@suse.de, xueyuan.chen21@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, surenb@google.com Content-Type: text/plain; charset="UTF-8" proc/pid/smaps_rollup can be read using the combination of RCU and VMA read locks, similar to proc/pid/{maps|smaps|numa_maps}. RCU is required to safely traverse the VMA tree and VMA lock stabilizes the VMA being processed and the pagetable walk. Note that we have to keep the logic to drop mmap_lock on contention because even when using per-VMA locks we might have to fall back to holding the mmap_lock. Running Paul's contention benchmark [1] shows considerable improvement both in median and in the worst case latencies: Execution command: run-proc-vs-map.sh --nsamples 20 --rawdata -- \ --busyduration 2 --procfile smaps_rollup Baseline: Median Minimum Maximum 0.174 0.161 2.553 0.174 0.164 2.663 0.174 0.165 2.664 0.174 0.166 2.679 0.174 0.167 2.691 0.174 0.168 2.704 0.174 0.169 2.729 0.174 0.172 2.741 0.174 0.174 2.745 0.174 0.174 2.755 0.174 0.175 2.790 0.174 0.177 2.809 0.174 0.179 3.096 0.174 0.183 3.144 0.174 0.184 3.158 0.174 0.185 3.175 0.174 0.185 4.568 0.174 0.198 4.821 0.174 0.214 5.143 0.174 0.251 5.220 Patched: Median Minimum Maximum 0.007 0.007 1.952 0.007 0.007 1.955 0.007 0.007 1.955 0.007 0.007 1.955 0.007 0.007 1.957 0.007 0.007 1.969 0.007 0.007 2.065 0.007 0.007 2.075 0.007 0.007 2.146 0.007 0.007 2.195 0.007 0.007 2.223 0.007 0.007 2.259 0.007 0.007 2.488 0.007 0.007 2.562 0.007 0.007 2.599 0.007 0.007 2.697 0.007 0.007 3.030 0.007 0.007 3.075 0.007 0.007 3.145 0.007 0.007 3.225 Remove now unused lock_ctx_mm() and move unlock_ctx_vma() next to unlock_ctx_mm() as they are logically related. Remove a long comment about 4 cases that we handle when dropping the mmap lock in the middle of VMA walk due to contention. The first 3 cases explained there are handled naturally and only case 4 needs to be handled in a special way, which is done in smap_gather_stats() by gathering stats from the portion of the VMA that has not yet been processed. For posterity, moving this comment here: After dropping the lock, there are four cases to consider. See the following example for explanation. +------+------+-----------+ | VMA1 | VMA2 | VMA3 | +------+------+-----------+ | | | | 4k 8k 16k 400k Suppose we drop the lock after reading VMA2 due to contention, then we get: last_vma_end = 16k 1) VMA2 is freed, but VMA3 exists: vma_next(vmi) will return VMA3. In this case, just continue from VMA3. 2) VMA2 still exists: vma_next(vmi) will return VMA3. In this case, just continue from VMA3. 3) No more VMAs can be found: vma_next(vmi) will return NULL. No more things to do, just break. 4) (last_vma_end - 1) is the middle of a vma (VMA'): vma_next(vmi) will return VMA' whose range contains last_vma_end. Iterate VMA' from last_vma_end. [1] https://github.com/paulmckrcu/proc-mmap_sem-test Signed-off-by: Suren Baghdasaryan --- fs/proc/task_mmu.c | 159 +++++++++++++++++++-------------------------- 1 file changed, 66 insertions(+), 93 deletions(-) diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 9a3c996c1d61..6fee40bdd736 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -130,28 +130,12 @@ static void release_task_mempolicy(struct proc_maps_private *priv) } #endif -static int lock_ctx_mm(struct proc_maps_locking_ctx *lock_ctx) -{ - int ret = mmap_read_lock_killable(lock_ctx->mm); - - if (!ret) - lock_ctx->mmap_locked = true; - - return ret; -} - static void unlock_ctx_mm(struct proc_maps_locking_ctx *lock_ctx) { mmap_read_unlock(lock_ctx->mm); lock_ctx->mmap_locked = false; } -static void reset_lock_ctx(struct proc_maps_locking_ctx *lock_ctx) -{ - lock_ctx->locked_vma = NULL; - lock_ctx->mmap_locked = false; -} - static void unlock_ctx_vma(struct proc_maps_locking_ctx *lock_ctx) { if (lock_ctx->locked_vma) { @@ -160,6 +144,12 @@ static void unlock_ctx_vma(struct proc_maps_locking_ctx *lock_ctx) } } +static void reset_lock_ctx(struct proc_maps_locking_ctx *lock_ctx) +{ + lock_ctx->locked_vma = NULL; + lock_ctx->mmap_locked = false; +} + static struct vm_area_struct *get_next_vma(struct proc_maps_private *priv, loff_t last_pos) { @@ -1384,12 +1374,14 @@ static int show_smap(struct seq_file *m, void *v) static int show_smaps_rollup(struct seq_file *m, void *v) { struct proc_maps_private *priv = m->private; + struct proc_maps_locking_ctx *lock_ctx = &priv->lock_ctx; + struct mm_struct *mm = lock_ctx->mm; struct mem_size_stats mss = {}; - struct mm_struct *mm = priv->lock_ctx.mm; + unsigned long last_vma_end = 0; + unsigned long vma_start = 0; struct vm_area_struct *vma; - unsigned long vma_start = 0, last_vma_end = 0; + loff_t pos = 0; int ret = 0; - VMA_ITERATOR(vmi, mm, 0); priv->task = get_proc_task(priv->inode); if (!priv->task) @@ -1400,89 +1392,66 @@ static int show_smaps_rollup(struct seq_file *m, void *v) goto out_put_task; } - ret = lock_ctx_mm(&priv->lock_ctx); - if (ret) - goto out_put_mm; - hold_task_mempolicy(priv); - vma = vma_next(&vmi); + rcu_read_lock(); + reset_lock_ctx(lock_ctx); + vma_iter_init(&priv->iter, mm, 0); + vma = proc_get_vma(m, &pos); if (unlikely(!vma)) goto empty_set; - vma_start = vma->vm_start; - do { - smap_gather_stats(priv, vma, &mss, vma->vm_start); + if (!IS_ERR(vma)) + vma_start = vma->vm_start; + + while (vma) { + unsigned long start; + + if (IS_ERR(vma)) { + ret = PTR_ERR(vma); + goto out_unlock; + } + + if (vma->vm_start < last_vma_end) { + /* + * After retaking the lock, already reported VMA grew + * or got merged with the next one and we found it + * again. Gather stats for the remaining portion by + * starting at last_vma_end. + */ + start = last_vma_end; + } else { + /* Found next unreported VMA, start from its beginning */ + start = vma->vm_start; + } + smap_gather_stats(priv, vma, &mss, start); last_vma_end = vma->vm_end; /* - * Release mmap_lock temporarily if someone wants to - * access it for write request. + * If the VMA lock is not taken, we hold the often contended + * mmap lock. This can happen if we had to fall back to the + * mmap lock. + * + * To relieve pressure, check if it is indeed contended, then + * temporarily release it. */ - if (mmap_lock_is_contended(mm)) { - vma_iter_invalidate(&vmi); - unlock_ctx_mm(&priv->lock_ctx); - ret = lock_ctx_mm(&priv->lock_ctx); - if (ret) { - release_task_mempolicy(priv); - goto out_put_mm; - } - + if (lock_ctx->mmap_locked && + mmap_lock_is_contended(lock_ctx->mm)) { + unlock_ctx_mm(lock_ctx); /* - * After dropping the lock, there are four cases to - * consider. See the following example for explanation. - * - * +------+------+-----------+ - * | VMA1 | VMA2 | VMA3 | - * +------+------+-----------+ - * | | | | - * 4k 8k 16k 400k - * - * Suppose we drop the lock after reading VMA2 due to - * contention, then we get: - * - * last_vma_end = 16k - * - * 1) VMA2 is freed, but VMA3 exists: - * - * vma_next(vmi) will return VMA3. - * In this case, just continue from VMA3. - * - * 2) VMA2 still exists: - * - * vma_next(vmi) will return VMA3. - * In this case, just continue from VMA3. - * - * 3) No more VMAs can be found: - * - * vma_next(vmi) will return NULL. - * No more things to do, just break. - * - * 4) (last_vma_end - 1) is the middle of a vma (VMA'): - * - * vma_next(vmi) will return VMA' whose range - * contains last_vma_end. - * Iterate VMA' from last_vma_end. + * Even though we previously fell back to mmap lock, + * we try taking VMA lock for the next VMA, since it + * might not be under modification. In the worst case + * we will fall back to mmap lock again. */ - vma = vma_next(&vmi); - /* Case 3 above */ - if (!vma) - break; - - /* Case 1 and 2 above */ - if (vma->vm_start >= last_vma_end) { - smap_gather_stats(priv, vma, &mss, vma->vm_start); - last_vma_end = vma->vm_end; - continue; - } - - /* Case 4 above */ - if (vma->vm_end > last_vma_end) { - smap_gather_stats(priv, vma, &mss, last_vma_end); - last_vma_end = vma->vm_end; - } + rcu_read_lock(); + reset_lock_ctx(lock_ctx); + /* Resume from the last position. */ + pos = last_vma_end; + vma_iter_init(&priv->iter, mm, pos); } - } for_each_vma(vmi, vma); + vma = proc_get_vma(m, &pos); + } empty_set: show_vma_header_prefix(m, vma_start, last_vma_end, 0, 0, 0, 0); @@ -1491,10 +1460,14 @@ static int show_smaps_rollup(struct seq_file *m, void *v) __show_smap(m, &mss, true); +out_unlock: + if (lock_ctx->mmap_locked) { + unlock_ctx_mm(lock_ctx); + } else { + unlock_ctx_vma(lock_ctx); + rcu_read_unlock(); + } release_task_mempolicy(priv); - unlock_ctx_mm(&priv->lock_ctx); - -out_put_mm: mmput(mm); out_put_task: put_task_struct(priv->task); -- 2.55.0.1007.g17ff1f9808-goog