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

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Date:   Fri, 21 Oct 2016 16:18:26 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Vivek Goyal <vgoyal@...hat.com>,
        "linux-unionfs@...r.kernel.org" <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>,
        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 Fri, Oct 21, 2016 at 12:30 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Fri, Oct 21, 2016 at 11:13 AM, Amir Goldstein <amir73il@...il.com> wrote:
>
>> I think it would be a good idea in general to stabilize the overlay ino/dev
>> throughout copy-up, same as Miklos suggested to do for directories, to
>> all files:
>> pure upper uses upper ino + overlayfs dev
>> non-pure upper uses lower ino + overlayfs dev
>
> Making st_ino, st_dev and d_ino behave consistently would be the next big step.
>
> The above scheme only works if lower and upper are on the same
> filesystem.  Otherwise there can be collisions between the lower and
> upper inode numbers.  Perhaps you meant:
>
> - pure upper uses upper ino + upper dev
> - non-pure upper uses lower ino + overlayfs dev
>
> It works for the single lower layer case, but again breaks if there
> are multiple lower layers.   And d_ino in a merged directory could
> still get us into trouble.  And find -xdev would not do what you'd
> expect with a "normal" filesystem.
>
> So there doesn't appear to be any easy solutions to this...
>

Not for the general case there isn't, but I was actually thinking of
the docker case
and there is a lot that can be done for the use case of lower and upper on the
same fs to make overlayfs more compliant.
Since it's quite a common use case, perhaps its worth the special treatment.

Amir.