Re: [PATCH 3/4 v2] fs: proc: use down_read_killable() in environ_read()

[Yang Shi]

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@xxxxxxxxxxxxxxxxx>
> > Suggested-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> 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);