Re: [POC/RFC PATCH] overlayfs: constant inode numbers

From: Amir Goldstein
Date: Tue Nov 29 2016 - 08:06:31 EST

On Tue, Nov 29, 2016 at 1:34 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Tue, Nov 29, 2016 at 12:16 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>> On Mon, Nov 28, 2016 at 12:56 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>> [...]
>>> 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().
>> Okay. In fact always using the handle looks like a better option
>> overall. File handle should be unique for the lifetime of the
>> filesystem, while inode numbers may be reused.
>> Biggest drawback of the file handle based redirects is exactly that:
>> makes backing up the overlay basically impossible, since file handles
>> won't work after a backup + restore. But as an optimization, in
>> addition to path based redirects it would work fine and provide a good
>> way to get the stable ino.
> Not sure that I understand what you are suggesting, but I would be happy
> to make the needed adjustments to redirect_fh per your request if you clarify
> what you mean. From what I understand:
> 1. If redirect_dir=fh (and supported by layers), store lower handle
> on dir copy up in new xattr OVL_XATTR_FH
> 2. In ovl_rename(), set OVL_XATTR_REDIRECT regardless of OVL_XATTR_FH
> 3. In ovl_lookup_single(), carry both d.redirct and d.redirect_fh to next layer
> 4. In ovl_lookup_layer(), lookup by handle first then by path
> Mind you that unlike OVL_XATTR_INO, OVL_XATTR_FH points to *next* inode
> rather then *lowest* inode, so it does not really improve anything wrt
> getting the
> stable inode. Stable inode of merged dir is available after lookup in
> oe->lowerstack[oe->numlower - 1].dentry->d_inode->i_ino
> regardless of whether lookup was by handle or by path or no redirect at all, so
> not sure what you meant by "... and provide a good way to get the stable ino."
> Either I managed to confuse you, or I am missing something?

Perhaps you meant for non-dir:

5. If redirect_dir=fh, *propagate* lowest-handle on non-dir copy up
6. In ovl_lookup() of non-dir, decode lowest-handle to set oe->ino

This is a good way to avoid stale OVL_XATTR_INO after backup+restore.