linux-kernel - Re: [POC/RFC PATCH] overlayfs: fix data inconsistency at copy up

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Date:   Fri, 21 Oct 2016 11:53:41 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Vivek Goyal <vgoyal@...hat.com>, Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-unionfs@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jeremy Eder <jeder@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Ratna Bolla <rbolla@...tworx.com>, Gou Rao <grao@...tworx.com>,
        Vinod Jayaraman <jv@...tworx.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [POC/RFC PATCH] overlayfs: fix data inconsistency at copy up

On Thu, Oct 20, 2016 at 11:54 PM, Vivek Goyal <vgoyal@...hat.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
> --
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