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

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Date:   Tue, 27 Feb 2018 08:59:06 -0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Alexey Dobriyan <adobriyan@...il.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 2/26/18 11:15 PM, Alexey Dobriyan wrote:
> 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 mean fix reading /proc/*/environ part.

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

There might be other places that need non-killable access_remote_vm(), 
like patch 4/4.

And, it sounds keeping access_remote_vm() semantic intact may prevent 
from confusion. The most people may get used to assume 
access_remote_vm() behaves like access_process_vm() except not inc mm 
reference count.

Thanks,
Yang

>
>> --- 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);