linux-unionfs.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Amir Goldstein <amir73il@gmail.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Zhihao Cheng <chengzhihao1@huawei.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	Daniel J Walsh <dwalsh@redhat.com>,
	Nalin Dahyabhai <nalin@redhat.com>
Subject: Re: [QUESTION] Why overlayfs cannot mounted as nfs_export and metacopy?
Date: Mon, 16 Aug 2021 21:44:11 +0300	[thread overview]
Message-ID: <CAOQ4uxjbQFTYof8L5YS9RTZ81hGHj3CgVBgF1jgXRdT6=1svwQ@mail.gmail.com> (raw)
In-Reply-To: <YRqt9u/P8VhOZCmL@redhat.com>

On Mon, Aug 16, 2021 at 9:27 PM Vivek Goyal <vgoyal@redhat.com> wrote:
>
> On Wed, Aug 11, 2021 at 09:51:09AM +0300, Amir Goldstein wrote:
> > On Tue, Aug 10, 2021 at 10:49 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> > >
> > > On Tue, Aug 10, 2021 at 07:17:23AM +0300, Amir Goldstein wrote:
> > > > On Tue, Aug 10, 2021 at 12:35 AM Vivek Goyal <vgoyal@redhat.com> wrote:
> > > > >
> > > > > On Sat, Aug 07, 2021 at 07:37:00PM +0300, Amir Goldstein wrote:
> > > > > > On Sat, Aug 7, 2021 at 2:05 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > > >
> > > > > > > On Sat, Aug 7, 2021 at 1:17 PM Zhihao Cheng <chengzhihao1@huawei.com> wrote:
> > > > > > > >
> > > > > > > > Hi, all.
> > > > > > > >
> > > > > > > > As title said. I wonder to know the reason for overlayfs mount failure
> > > > > > > > with '-o nfs_export=on,metacopy=on'.
> > > > > > > >
> > > > > > > > I modified kernel to enable these two options 'on',  it looks like that
> > > > > > > > overlayfs can still work fine under nfs_v4.
> > > > > > > >
> > > > > > > > Besides, I can get no more information about the reason from source
> > > > > > > > code, maybe I missed something.
> > > > > > > >
> > > > > > >
> > > > > > > It's because ovl_obtain_alias() (decoding a disconnected non-dir file handle)
> > > > > > > does not know how to construct a metacopy overlayfs inode.
> > > > > > >
> > > > > > > Maybe Vivek will be able to point you to the discussion that lead to making
> > > > > > > the features mutually exclusive.
> > > > > > >
> > > > > > > I don't remember any other reason.
> > > > > > >
> > > > > >
> > > > > > I remembered some more details...
> > > > > >
> > > > > > I think the main complication discussed w.r.t decoding a metacopy
> > > > > > inode was for the case where ovl_inode_lowerdata() differs from
> > > > > > ovl_inode_lower().
> > > > > >
> > > > > > If we had a weaker variant of metacopy (e.g. metacopy=upper) that
> > > > > > only allows creating and following metacopy inodes in the upper layer,
> > > > > > it would have been simpler to implement ovl_obtain_alias().
> > > > > >
> > > > > > Specifically, when ofs->numlayer == 2 (single lower layer), there can
> > > > > > be no valid metacopy inodes in the lower layer, so that configuration
> > > > > > should also be rather easy to support.
> > > > >
> > > > > Hi Amir,
> > > > >
> > > > > /me does not understand well the notion of disconnected dentries and
> > > > >  how nfs export stuff works. So please bear with my stupid questions.
> > > >
> > > > No stupid questions ;-)
> > > >
> > > > Without getting into the hairy details of nfs export there are a few basic
> > > > things to consider:
> > > > - A file handle does not encode the path, only an inode identifier
> > > > - A non-directory inode may have multiple paths (hardlinks)
> > > > - Most filesystems do not store path information in inode on-disk for
> > > >   non-directory inode (the ".." entry stores the path for a directory)
> > > > - When filesystem is asked to decode a file handle and does not find the
> > > >   inode in question in inode cache nor a dentry in dcache, the only resort
> > > >   is to instantiate a "disconnected" dentry with unknown path
> > > > - Later "normal" lookup() by path that resolves to the same inode, does not
> > > >   make that "disconnected" dentry connected. Istead, lookup() instantiates
> > > >   another connected dentry "alias" to the same inode
> > > >
> > > > All this has some implications when enabling nfs_export for overlayfs:
> > > > 1. ovl_obtain_dentry() needs to be able to cope with a disconnected
> > > >     'real' dentry
> > > > 2. Since ovl_obtain_dentry() cannot assume to know the path of the
> > > >     'real' dentry, it needs to know how to instantiate a disconnected
> > > >     overlayfs dentry
> > > > 3. Other overlayfs code needs to be able to cope with a disconnected
> > > >     overlayfs dentry (for example, copy up only to index)
> > >
> > > Hi Amir,
> > >
> > > Thanks for giving a summary. It helps a lot.
> > >
> > > >
> > > > >
> > > > > I am wondering why a lower inode can't be metacopy inode. For the
> > > > > normal lookup case, we can lookup in all lower layers and figure out
> > > > > which is actual data inode and which inodes are metacopy inodes.
> > > > >
> > > > > For the case of disconnected dentry, we probably can't do lookup. So
> > > > > are calling underlying filesystem to decode. (Using origin?). If yes,
> > > > > will intermediate lower not have origin xattr which we can use
> > > > > to follow the complete lower chain and reconstruct all real lower
> > > > > dentries and use lower data dentry and latest lower meatacopy dentry
> > > > > (in the same way we do as for lookup).
> > > >
> > > > We can do that. I did not say we cannot.
> > > > I just said it would be simpler if we can avoid this complication
> > > > and I listed the guidelines for the "simple" implementation.
> > > >
> > > > But beyond the complexity, what is the benefit?
> > > > I was under the impression that container manager do not know how
> > > > to build images with metacopy, so what are the chances of actually
> > > > seeing metacopy in middle layers in the wild?
> > >
> > > Sure, we don't put metacopy inodes into portable images. But I thougt
> > > this could be part of a lower directory on same system. For example,
> > > docker devicemapper driver used to take an image and explode that
> > > on a thin volume. Then it will take a snaphost, modify some files
> > > and prefix that intermediate state with "-init". And then containers
> > > will use this "-init" as base for container rootfs and take snapshot
> > > of this.
> > >
> > > I am not sure if container managers are doing this for overlayfs
> > > or not on same system. But I will not be surprised if somebody
> > > decides to do that. That's is change some metadata in image
> > > (which triggers metacopy) and then use upper layer as lower layer
> > > for container rootfs.
> > >
> > > [ CC Dan Walsh and Nalin Dahyabhai ]
> > >
> > > Dan, Nalin, I think now metacopy feature is being used in podman and
> > > container/storage. Do we ever create lower layers in such a way that
> > > metacopy inodes can be present in lower layers?
> > >
> > > >
> > > > IOW, if we implemented metacopy=upper (only allow metacopy in
> > > > upper layer), would it be sufficient for the use cases that need to enable
> > > > nfs_export?
> > >
> > > IMHO, it will be good if we don't add one more variation to metacopy
> > > option. Even if container managers will find it hard to ensure that
> > > there are no metacopy inodes in any of the lower layers. And will
> > > find it difficult to use this option.
> > >
> > > IIRC, while adding metacopy option, you had mentioned that it is possible
> > > that intermediate layers have metacopy inodes. So I changed implementation
> > > to take care of it. :-) And now you are the one suggesting not allowing
> > > metacopy inodes in lower layers with nfs_export. :-)
> > >
> >
> > Maybe. It wouldn't be the first time ;-)
> > IIRC the idea to lookup metacopy by path+redirect was Miklos' not mine.
> > I was thinking in terms of following by origin back then.
>
> Yes. I think we had lot of discussion on this. You wanted to use origin.
> And I preferred not to use it primarily because I wanted to make use
> of metacopy even without index enabled also because did not want
> to depend on lower layers supporting nfs_export. So lookup approach
> seemed simpler and using origin did not seem necessary.
>
> >
> > The eventual code in ovl_lookup() follows upper by origin and all layers
> > by path+redirect and only checks for agreement of origin and redirect from
> > upper layer.
> >
> > Implementing "follow metacopy in middle layers by origin" in ovl_obtain_alias()
> > is possible and even not too complicated, but it would be inconsistent
> > with ovl_lookup() and if somebody mangaled with lower layers (i.e. rename
> > lower file), decode file handle can result in a different inode than the encoded
> > one.
> >
> > In that case we have several options:
> > 1. Whatever - documentation already claims that modifying lower layer
> >     after using index/metacopy results in undefined behavior
> > 2. Fortify ovl_lookup() - also follow origin from middle layers and check
> >     for agreement with follow by path+redirect
>
> The idea of fortifying that lower we found matches origin sounds
> reasonable to me. I thought that's what verify_lower was doing. But may
> be in limited cases.
>
> We probably will have to again make it opt-in so that we don't break
> some existing use cases (not opting in for origin created so many
> issues on squashfs that I have lost track of all the issues and fixes :-))
>
> >
> > > If given a choice, I would prefer that we support metacopy inodes
> > > in lower layers as well with nfs_export and not create a new option
> > > metacopy=upper.
> > >
> >
> > Certainly. I agree with that POV. We just need to understand the
> > consequences.
> >
> > I personally have no problem with the "Whatever" option above.
> > And the "Fortify" option isn't that complex either.
> >
> > I just thought that if Zhihao Cheng wanted to start with minimal
> > testable implementation, that limiting nfs_export+metacopy to a single
> > lower layer, may be an easier start.
> >
> > Overlayfs nfs_export is quite likely being used in OpenWrt and IIUC,
> > OpenWrt uses a single (squashfs) lower layer.
> >
> > Therefore, enabling metacopy for the single lower layer case may be
> > valuable to some actual users and not only as a stepping stone before a
> > complete solution.
>
> I agree that metacopy=upper as it is will be useful to some people
> in some configurations. Do you really want to support two options.
> More options lead to more confusion for users and more configurations can
> be harder to support. So if it was me, I would rather target just making
> metacopy=on/off work with nfs_export. Given I am not doing the actual
> work, I can have only so much say.. :-)
>

Maybe I was not clear, so let me try again.
The most simple thing to implement would be to allow
nfs_export=on along with metacopy=on when there is one single lower layer.

Assuming that configuration is useful for whoever intends to implement it.
From there we can gather some experience and move on to supporting
multi lower layers if needed.

Thanks,
Amir.

  reply	other threads:[~2021-08-16 18:44 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-07 10:17 [QUESTION] Why overlayfs cannot mounted as nfs_export and metacopy? Zhihao Cheng
2021-08-07 11:05 ` Amir Goldstein
2021-08-07 16:37   ` Amir Goldstein
2021-08-09  9:21     ` Zhihao Cheng
2021-08-09 21:35     ` Vivek Goyal
2021-08-10  4:17       ` Amir Goldstein
2021-08-10 19:48         ` Vivek Goyal
2021-08-10 20:00           ` Vivek Goyal
2021-08-11  6:51           ` Amir Goldstein
2021-08-16 18:27             ` Vivek Goyal
2021-08-16 18:44               ` Amir Goldstein [this message]
2021-08-16 19:00                 ` Vivek Goyal
2021-08-09 19:56   ` Vivek Goyal

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAOQ4uxjbQFTYof8L5YS9RTZ81hGHj3CgVBgF1jgXRdT6=1svwQ@mail.gmail.com' \
    --to=amir73il@gmail.com \
    --cc=chengzhihao1@huawei.com \
    --cc=dwalsh@redhat.com \
    --cc=linux-unionfs@vger.kernel.org \
    --cc=miklos@szeredi.hu \
    --cc=nalin@redhat.com \
    --cc=vgoyal@redhat.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).