linux-kernel - Re: [PATCH v2 3/3] fs/proc/task_mmu: execute PROCMAP

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Message-ID: <a2fca13d-87bd-4eb3-b673-46c538f46e66@suse.cz>
Date: Tue, 5 Aug 2025 16:20:47 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc: Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com, david@...hat.com,
 peterx@...hat.com, jannh@...gle.com, hannes@...xchg.org, mhocko@...nel.org,
 paulmck@...nel.org, shuah@...nel.org, adobriyan@...il.com,
 brauner@...nel.org, josef@...icpanda.com, yebin10@...wei.com,
 linux@...ssschuh.net, willy@...radead.org, osalvador@...e.de,
 andrii@...nel.org, ryan.roberts@....com, christophe.leroy@...roup.eu,
 tjmercier@...gle.com, kaleshsingh@...gle.com, aha310510@...il.com,
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 3/3] fs/proc/task_mmu: execute PROCMAP_QUERY ioctl
 under per-vma locks

On 8/5/25 1:15 AM, Suren Baghdasaryan wrote:
> Utilize per-vma locks to stabilize vma after lookup without taking
> mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is
> contended, we fall back to mmap_lock but take it only momentarily
> to lock the vma and release the mmap_lock. In a very unlikely case
> of vm_refcnt overflow, this fall back path will fail and ioctl is
> done under mmap_lock protection.
> 
> This change is designed to reduce mmap_lock contention and prevent
> PROCMAP_QUERY ioctl calls from blocking address space updates.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  fs/proc/task_mmu.c | 81 +++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 65 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 843577aa7a32..1d06ecdbef6f 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -517,28 +517,78 @@ static int pid_maps_open(struct inode *inode, struct file *file)
>  		PROCMAP_QUERY_VMA_FLAGS				\
>  )
>  
> -static int query_vma_setup(struct mm_struct *mm)
> +#ifdef CONFIG_PER_VMA_LOCK
> +
> +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
>  {
> -	return mmap_read_lock_killable(mm);
> +	lock_ctx->locked_vma = NULL;
> +	lock_ctx->mmap_locked = false;
> +
> +	return 0;
>  }
>  
> -static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma)
> +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
>  {
> -	mmap_read_unlock(mm);
> +	if (lock_ctx->mmap_locked)
> +		mmap_read_unlock(lock_ctx->mm);
> +	else
> +		unlock_vma(lock_ctx);
>  }
>  
> -static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr)
> +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> +						     unsigned long addr)
>  {
> -	return find_vma(mm, addr);
> +	struct vm_area_struct *vma;
> +	struct vma_iterator vmi;
> 

Hm I think we can reach here with lock_ctx->mmap_locked being true via
"goto next_vma" in query_matching_vma(). In that case we should just
"return find_vma()" and doing the below is wrong, no?

> +	unlock_vma(lock_ctx);
> +	rcu_read_lock();
> +	vma_iter_init(&vmi, lock_ctx->mm, addr);
> +	vma = lock_next_vma(lock_ctx->mm, &vmi, addr);
> +	rcu_read_unlock();
> +
> +	if (!IS_ERR_OR_NULL(vma)) {
> +		lock_ctx->locked_vma = vma;
> +	} else if (PTR_ERR(vma) == -EAGAIN) {
> +		/* Fallback to mmap_lock on vma->vm_refcnt overflow */
> +		mmap_read_lock(lock_ctx->mm);
> +		vma = find_vma(lock_ctx->mm, addr);
> +		lock_ctx->mmap_locked = true;
> +	}
> +
> +	return vma;
>  }
>