linux-kernel - Re: [PATCH 3/4 v2] fs: proc: use down_read_killable() in environ

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Message-ID: <20180227071536.GA5234@avx2>
Date:   Tue, 27 Feb 2018 10:15:36 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, mingo@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4 v2] fs: proc: use down_read_killable() in
 environ_read()

On Tue, Feb 27, 2018 at 08:25:50AM +0800, Yang Shi wrote:
> Like reading /proc/*/cmdline, it is possible to be blocked for long time
> when reading /proc/*/environ when manipulating large mapping at the mean
> time. The environ reading process will be waiting for mmap_sem become
> available for a long time then it may cause the reading task hung.
> 
> Convert down_read() and access_remote_vm() to killable version.
> 
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> Suggested-by: Alexey Dobriyan <adobriyan@...il.com>

Ehh, bloody tags.
I didn't suggest _killable() variants, they're quite ugly because API
multiplies. access_remote_vm() could be converted to down_read_killable().

> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -933,7 +933,9 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>  	if (!mmget_not_zero(mm))
>  		goto free;
>  
> -	down_read(&mm->mmap_sem);
> +	ret = down_read_killable(&mm->mmap_sem);
> +	if (ret)
> +		goto out_mmput;
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
>  	up_read(&mm->mmap_sem);
> @@ -950,7 +952,8 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>  		max_len = min_t(size_t, PAGE_SIZE, count);
>  		this_len = min(max_len, this_len);
>  
> -		retval = access_remote_vm(mm, (env_start + src), page, this_len, 0);
> +		retval = access_remote_vm_killable(mm, (env_start + src),
> +						page, this_len, 0);