From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0D92250B41B for ; Fri, 18 Sep 2026 15:45:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789746309; cv=none; b=gTDWYjh6PH2gzEkGIwVvyz1ztpBYHekEe6ZPkZosgQWSvQFfKX2FtFJqeFBPBMG56G94jiZVjn0MjX8keUhNYIhUVn3Rv7cpovpaG2WGkXzMGdZTo3tXXlnP7Y9IdzHoc7Nr6gexgzZfm4k+1TeQfSWuuzCyZTXAXejIi7mbROg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789746309; c=relaxed/simple; bh=7rzr4eyJlnvLolgSRYMAVOIzuf7BJ57VyXME60aYlu4=; h=Date:To:From:Subject:Message-Id; b=fRqkYmrw+dtcTsMMvoiqtkGQE8Dr+p6wFG9eOLQQQcNvcVya0PxwABo+R2V+W2HhjbB2M+ed+zd/UF53T/NyJmLN+IuBOOPRuVYULO1UIovXuzBml/n+K8hWrcV1FdNRvbRvUOxxA1b2cGcAVCc6tsiG0nVDYNgr2JR5P7FO+ag= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=n1D2vGuS; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="n1D2vGuS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D4BD41F00898; Fri, 18 Sep 2026 15:45:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1789746306; bh=MjKeeHBwX8Nr5qM2j4JkLBssuRgxdK3AaNkHDqYWPQQ=; h=Date:To:From:Subject; b=n1D2vGuSXsQHf4d1Qjpc+BA0seEUIOl/7SzfAl8qJEo90D1A6WCHkCAjWa8j/kxTd 5JpBzkOvtSm4EIg8jsMQKbOhD8LZaYe+itTxQMmj10xy+LdloExhfJJtYXiaIiRwft d7Xw+bdWMmdGNa+a9vFk4Q5QnpicGXzWJzgZbE8c= Date: Fri, 18 Sep 2026 08:45:06 -0700 To: mm-commits@vger.kernel.org,surenb@google.com,akpm@linux-foundation.org From: Andrew Morton Subject: + proc-task_mmu-read-proc-pid-smaps_rollup-under-per-vma-lock.patch added to mm-new branch Message-Id: <20260918154506.D4BD41F00898@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: proc/task_mmu: read proc/pid/smaps_rollup under per-vma lock has been added to the -mm mm-new branch. Its filename is proc-task_mmu-read-proc-pid-smaps_rollup-under-per-vma-lock.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/proc-task_mmu-read-proc-pid-smaps_rollup-under-per-vma-lock.patch This patch will later appear in the mm-new branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Note, mm-new is a provisional staging ground for work-in-progress patches, and acceptance into mm-new is a notification for others take notice and to finish up reviews. Please do not hesitate to respond to review feedback and post updated versions to replace or incrementally fixup patches in mm-new. The mm-new branch of mm.git is not included in linux-next If a few days of testing in mm-new is successful, the patch will me moved into mm.git's mm-unstable branch, which is included in linux-next Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Suren Baghdasaryan Subject: proc/task_mmu: read proc/pid/smaps_rollup under per-vma lock Date: Fri, 18 Sep 2026 08:33:17 -0700 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. Link: https://lore.kernel.org/20260918153318.758387-7-surenb@google.com Link: https://github.com/paulmckrcu/proc-mmap_sem-test [1] Signed-off-by: Suren Baghdasaryan Signed-off-by: Andrew Morton Reviewed-by: Lorenzo Stoakes (ARM) Cc: David Hildenbrand (Arm) Cc: Jann Horn Cc: Liam R. Howlett (Oracle) Cc: Matthew Wilcox (Oracle) Cc: "Paul E . McKenney" Cc: Pedro Falcato Cc: Usama Arif Cc: Vlastimil Babka --- fs/proc/task_mmu.c | 159 +++++++++++++++++-------------------------- 1 file changed, 63 insertions(+), 96 deletions(-) --- a/fs/proc/task_mmu.c~proc-task_mmu-read-proc-pid-smaps_rollup-under-per-vma-lock +++ a/fs/proc/task_mmu.c @@ -130,30 +130,12 @@ static void release_task_mempolicy(struc } #endif -#ifdef CONFIG_PROC_PAGE_MONITOR -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; -} -#endif - 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) { @@ -162,6 +144,12 @@ static void unlock_ctx_vma(struct proc_m } } +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) { @@ -1406,12 +1394,14 @@ static int show_smap(struct seq_file *m, 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) @@ -1422,90 +1412,63 @@ static int show_smaps_rollup(struct seq_ 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); + if (!IS_ERR(vma)) + vma_start = vma->vm_start; + + while (vma) { + 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. + */ + smap_gather_stats_range(priv, vma, &mss, last_vma_end); + } else { + /* Found next unreported VMA, start from its beginning */ + smap_gather_stats(priv, vma, &mss); + } 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); - last_vma_end = vma->vm_end; - continue; - } - - /* Case 4 above */ - if (vma->vm_end > last_vma_end) { - smap_gather_stats_range(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); @@ -1514,10 +1477,14 @@ empty_set: __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); _ Patches currently in -mm which might be from surenb@google.com are proc-task_mmu-remove-unnecessary-helpers.patch proc-task_mmu-remove-unnecessary-inlines-in-function-definitions.patch proc-task_mmu-clarify-shmem-mapping-walk-conditions-in-smap_gather_stats.patch proc-task_mmu-remove-special-casing-of-smap_gather_stats-start-parameter.patch proc-task_mmu-change-proc_get_vma-to-stop-returning-gate-vma-at-the-end.patch proc-task_mmu-read-proc-pid-smaps_rollup-under-per-vma-lock.patch selftests-proc-add-proc-pid-smaps_rollup-tearing-tests.patch