Date:   Mon, 28 Nov 2016 20:02:36 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [POC/RFC PATCH] overlayfs: constant inode numbers

On Mon, Nov 28, 2016 at 1:56 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Mon, Nov 28, 2016 at 12:35 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
>> On Mon, Nov 28, 2016 at 10:10 AM, Amir Goldstein <amir73il@...il.com> wrote:
>>>>> @@ -258,12 +268,12 @@ static int ovl_copy_up_locked(struct den
>>>>>         if (err)
>>>>>                 goto out_cleanup;
>>>>>
>>>>> -       inode_lock(newdentry->d_inode);
>>>>>         err = ovl_set_attr(newdentry, stat);
>>>>> -       inode_unlock(newdentry->d_inode);
>>>>>         if (err)
>>>>>                 goto out_cleanup;
>>>>>
>>>>> +       ovl_dentry_set_ino(dentry, stat->ino);
>>>>> +
>>>>
>>>>
>>>
>>> Shouldn't we propagate stored ino all the way
>>> from lowest layer to preserve ino across layer recycling?
>>
>> Since OVL_XATTR_INO is set every time we copy-up, layer recycling
>> should work fine.
>>
>
> Right. I knew it should be there somewhere, but miss-read the copy up code.
>
>> Exception is overlay root, where there's no copy-up, so no
>> preservation of ino.  Not sure what the right solution there is.
>>
>>>
>>> Specifically, shouldn't ino of merged dir expose the lower most ino?
>>
>> It should.
>>
>>
>>>>> @@ -353,11 +354,45 @@ static struct ovl_dir_cache *ovl_cache_g
>>>>>         return cache;
>>>>>  }
>>>>>
>>>>> +static int ovl_cache_entry_update_ino(struct dentry *dir,
>>>>> +                                     struct ovl_cache_entry *p)
>>>>> +
>>>>> +{
>>>>> +       struct dentry *this;
>>>>> +       struct dentry *base = ovl_dentry_at_idx(dir, p->idx);
>>>>> +
>>>>> +       if (p->name[0] == '.') {
>>>>> +               if (p->len == 1) {
>>>>> +                       this = dget(base);
>>>>> +                       goto get;
>>>>> +               }
>>>>> +               if (p->len == 2 && p->name[1] == '.') {
>>>>> +                       /* ♫ we shall not be moved */

I don't think music notes are allowed in comments outside the audio
subsystem ;-)

>>>>> +                       this = dget(ovl_dentry_real(dir->d_parent));
>>>>> +                       goto get;
>>>>> +               }
>>>>> +       }
>>>>> +       this = lookup_one_len_unlocked(p->name, base, p->len);
>>>>> +       if (IS_ERR(this)) {
>>>>> +               pr_err("overlay: failed to look up (%s) for ino (%i)\n",
>>>>> +                      p->name, (int) PTR_ERR(this));
>>>>> +               return -EIO;
>>>>> +       }
>>>>>
>>>>> +       get:

indentation of goto label.

>>>>> +       p->ino = p->orig_ino;
>>>>> +       ovl_get_ino(this, &p->ino);
>>>
>>> I may be way off here, but why do you need to lookup entry and get ino
>>> from xattr at all? Wouldn't it be easier to not add the entry to the list if
>>> it was copied up and rely on the fact that it will be added to list in iter
>>> of lower layer with original ino with no extra effort?
>>
>> What about renamed entries?
>
> Right. I completely missed out on the rename case..
>
>> What about opaque ones?
>
> That's exactly the point of OVL_XATTR_INO IFF !OVL_XATTR_OPAQUE
> If you have OVL_XATTR_INO means entry cannot be opaque, so it should
> be safe to skip it
>
>>
>> I do hope we can optimize directory reading, because doing lookup and
>> getxattr for all entries is going to hurt...
>>
>
> Possibly silly question:
> Do you know if programs really rely of d_ino from getdents to be sane/non-zero?
> And what are the implications of overlayfs readdir not exporting the real d_ino?
>
> For example, findutils seems to be ok with zero d_ino and just uses non-zero
> d_ino for optimization:
>
> commit 2bf001636e66789560ef1d2509c117f78b6cd06f
> Author: James Youngman <jay@....org>
> Date:   Sat Mar 7 20:16:49 2009 +0000
>
>     Optimise away calls to stat if all we need is the inode number (bug #24342).
>
>
>>> For that matter, I like the fact that every copied up entry will be explicitly
>>> marked with OVL_XATTR_INO. In a way, it is the opposite of
>>> OVL_XATTR_OPAQUE and if the former becomes a standard, the latter
>>> will become redundant. Arguably, it is preferred to mark the copy ups
>>> as special case rather then the pure upper files, which can then stay 'pure'.
>>
>> Maybe.
>>
>>
>>>>> @@ -502,7 +549,8 @@ static int ovl_dir_open(struct inode *in
>>>>>                 return PTR_ERR(realfile);
>>>>>         }
>>>>>         od->realfile = realfile;
>>>>> -       od->is_real = !OVL_TYPE_MERGE(type);
>>>>> +//     od->is_real = !OVL_TYPE_MERGE(type);
>>>>> +       od->is_real = false;
>>>
>>>
>>> A major change of framing would be to treat regular file entries as merged
>>> if they have been ever copied up and opaque only if they are pure upper.
>>> Same as dirs.
>>>
>>> This would also allow keeping ino stable across rename/redirect of regular
>>> files. Not sure if any programs rely on that??
>>
>> We do keep ino stable across rename.  We don't keep ino stable across
>> copy-up.  That's what this patch is trying to address.
>>
>> You are saying that we should have redirects for non-dir and drop
>> OVL_XATTR_INO?  That's another option, but it doesn't look like it
>> would simplify things...
>>
>
> Well, not sure if you noticed my redirect_fh (rediect by file handle) work.
> If differs from redirect by path in 2 major ways:
> 1. Like OVL_XATTR_INO, redirect is set on copy up (but only for dirs)
> 2. Lookup is much simpler (and most likely faster) then full path lookup
>
> It would be trivial to set oe->ino of merged dir from lower most entry in
> ovl_lookup().
>
> So while I cannot justify non-dir redirect in favor of OVL_XATTR_INO,
> I do see a big value for redirect by file handle for directories, which can
> provide the non-readdir part of stable directory inode as by-product.
>
>> Thanks for the revirew.
>>
>> Pushed patch to #overlayfs-constino (needs work but it's worth testing).
>>
>
> Thanks. I'll get to testing this later this week and will do my best to draft a
> quick xfstest for this as well.
>

So far so good, passed xfstest/pjdfstests/unionmount (--ov=0/10).
The stable st_dev change actually broke some assumptions made by
unionmount-testsuite so test needed some fixes to check_layer()
fixes available at:
https://github.com/amir73il/unionmount-testsuite.git #overlayfs-devel

commit f4d2bee (Test lower/upper on the same underlying fs) has
instructions for running unionmount tests over same underlaying fs.

Tested-by: Amir Goldstein <amir73il@...il.com>

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