lkml.org 
[lkml]   [2016]   [Oct]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [POC/RFC PATCH] overlayfs: fix data inconsistency at copy up
On Thu, Oct 20, 2016 at 11:54 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> On Thu, Oct 20, 2016 at 04:46:30PM -0400, Vivek Goyal wrote:
>
> [..]
>> > +static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *to)
>> > +{
>> > + struct file *file = iocb->ki_filp;
>> > + bool isupper = OVL_TYPE_UPPER(ovl_path_type(file->f_path.dentry));
>> > + ssize_t ret = -EINVAL;
>> > +
>> > + if (likely(!isupper)) {
>> > + const struct file_operations *fop = ovl_real_fop(file);
>> > +
>> > + if (likely(fop->read_iter))
>> > + ret = fop->read_iter(iocb, to);
>> > + } else {
>> > + struct file *upperfile = filp_clone_open(file);
>> > +
>>
>> IIUC, every read of lower file will call filp_clone_open(). Looking at the
>> code of filp_clone_open(), I am concerned about the overhead of this call.
>> Is it significant? Don't want to be paying too much of penalty for read
>> operation on lower files. That would be a common case for containers.
>>
>
> Looks like I read the code in reverse. So if I open a file read-only,
> and if it has not been copied up, I will simply call read_iter() on
> lower filesystem. But if file has been copied up, then I will call
> filp_clone_open() and pay the cost. And this will continue till this
> file is closed by caller.
>

I wonder if that cost could be reduced by calling replace_fd() or
some variant of it to install the cloned file onto the rofd after the
first access??

> When file is opened again, by that time it is upper file and we will
> install real fop in file (instead of overlay fop).
>
> Vivek
> --
> To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html

\
 
 \ /
  Last update: 2016-10-21 10:54    [W:3.728 / U:1.324 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site