Re: [PATCH v2 5/6] proc: use down_read_killable mmap_sem for /proc/pid/map_files

[Andrei Vagin <avagin@gmail.com>]

On Sun, Jun 09, 2019 at 01:09:00PM +0300, Konstantin Khlebnikov wrote:
> Do not stuck forever if something wrong.
> Killable lock allows to cleanup stuck tasks and simplifies investigation.

This patch breaks the CRIU project, because stat() returns EINTR instead
of ENOENT:

[root@fc24 criu]# stat /proc/self/map_files/0-0
stat: cannot stat '/proc/self/map_files/0-0': Interrupted system call

Here is one inline comment with the fix for this issue.

> 
> It seems ->d_revalidate() could return any error (except ECHILD) to
> abort validation and pass error as result of lookup sequence.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Reviewed-by: Roman Gushchin <guro@fb.com>
> Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
> Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>

It was nice to see all four of you in one place :).

> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
>  fs/proc/base.c |   27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 9c8ca6cd3ce4..515ab29c2adf 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1962,9 +1962,12 @@ static int map_files_d_revalidate(struct dentry *dentry, unsigned int flags)
>  		goto out;
>  
>  	if (!dname_to_vma_addr(dentry, &vm_start, &vm_end)) {
> -		down_read(&mm->mmap_sem);
> -		exact_vma_exists = !!find_exact_vma(mm, vm_start, vm_end);
> -		up_read(&mm->mmap_sem);
> +		status = down_read_killable(&mm->mmap_sem);
> +		if (!status) {
> +			exact_vma_exists = !!find_exact_vma(mm, vm_start,
> +							    vm_end);
> +			up_read(&mm->mmap_sem);
> +		}
>  	}
>  
>  	mmput(mm);
> @@ -2010,8 +2013,11 @@ static int map_files_get_link(struct dentry *dentry, struct path *path)
>  	if (rc)
>  		goto out_mmput;
>  
> +	rc = down_read_killable(&mm->mmap_sem);
> +	if (rc)
> +		goto out_mmput;
> +
>  	rc = -ENOENT;
> -	down_read(&mm->mmap_sem);
>  	vma = find_exact_vma(mm, vm_start, vm_end);
>  	if (vma && vma->vm_file) {
>  		*path = vma->vm_file->f_path;
> @@ -2107,7 +2113,10 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
>  	if (!mm)
>  		goto out_put_task;
>  
> -	down_read(&mm->mmap_sem);
> +	result = ERR_PTR(-EINTR);
> +	if (down_read_killable(&mm->mmap_sem))
> +		goto out_put_mm;
> +

	result = ERR_PTR(-ENOENT);

>  	vma = find_exact_vma(mm, vm_start, vm_end);
>  	if (!vma)
>  		goto out_no_vma;
> @@ -2118,6 +2127,7 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
>  
>  out_no_vma:
>  	up_read(&mm->mmap_sem);
> +out_put_mm:
>  	mmput(mm);
>  out_put_task:
>  	put_task_struct(task);
> @@ -2160,7 +2170,12 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
>  	mm = get_task_mm(task);
>  	if (!mm)
>  		goto out_put_task;
> -	down_read(&mm->mmap_sem);
> +
> +	ret = down_read_killable(&mm->mmap_sem);
> +	if (ret) {
> +		mmput(mm);
> +		goto out_put_task;
> +	}
>  
>  	nr_files = 0;
>