linux-kernel - Re: [PATCH 3/3] ovl: redirect on rename-dir

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Date:   Wed, 26 Oct 2016 13:26:45 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        Guillem Jover <guillem@...ian.org>,
        Raphael Hertzog <hertzog@...ian.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] ovl: redirect on rename-dir

On Tue, Oct 25, 2016 at 2:49 PM, Amir Goldstein <amir73il@...il.com> wrote:

>> @@ -880,31 +913,34 @@ static int ovl_rename(struct inode *olddir, struct dentry *old,
>>         if (WARN_ON(olddentry->d_inode == newdentry->d_inode))
>>                 goto out_dput;
>>
>> -       if (is_dir && !old_opaque && ovl_lower_positive(new)) {
>> -               err = ovl_set_opaque(olddentry);
>> -               if (err)
>> -                       goto out_dput;
>> -               ovl_dentry_set_opaque(old, true);
>> +       if (is_dir) {
>> +               if (ovl_type_merge_or_lower(old)) {
>> +                       err = ovl_set_redirect(old);
>
> There is a fair chance of getting ENOSPC/EDQUOT here and confuse user space.
> Would it be better to convert these non fatal errors with EXDEV, so
> user space will
> gracefully fallback to recursive rename/clone/copy?

Recursive copy up will surely consume more space than an xattr?

>> @@ -162,6 +223,23 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
>>                 stack[ctr].dentry = this;
>>                 stack[ctr].mnt = lowerpath.mnt;
>>                 ctr++;
>> +
>> +               if (!stop && i != poe->numlower - 1 &&
>> +                   d_is_dir(this) && ovl_redirect_dir(dentry->d_sb)) {
>> +                       err = ovl_check_redirect(this, &redirect);
>> +                       if (err)
>> +                               goto out_put;
>> +
>> +                       if (redirect && poe != dentry->d_sb->s_root->d_fsdata) {
>> +                               poe = dentry->d_sb->s_root->d_fsdata;
>> +
>
> Now you are about to continue looping until new value of poe->numlower,
> which is >= then olf value of poe->numlower, but 'stack' was allocated
> according to old value of poe->numlower, so aren't you in danger of
> overflowing it?
>
> Please add a comment to explain the purpose of this loop rewind.

We are jumping to a stack possibly wider than the current one and need
to find the layer where to continue the downward traversal.  I'll add
the comment.

BTW I don't remember having tested this, so it might possibly be
buggy.  Automatic multi-layer testing would really be good.  What we
basically need is:

 - create normal (two layer) overlay (with interesting constructs,
whiteout, opaque dir, redirect)
 - umount
 - create three layer overlay where the two lower layers come from the
previous upper/lower layers
 - do more interesting things

There's one such test in xfstests but it would be good to have more.

Thanks,
Miklos