Re: [PATCH v3 0/6] Composefs: an opportunistically sharing verified image filesystem

[Giuseppe Scrivano <gscrivan@redhat.com>]

Christian Brauner <brauner@kernel.org> writes:

> On Wed, Jan 25, 2023 at 02:46:59PM +0200, Amir Goldstein wrote:
>> > >
>> > > Based on Alexander's explanation about the differences between overlayfs
>> > > lookup vs. composefs lookup of a regular "metacopy" file, I just need to
>> > > point out that the same optimization (lazy lookup of the lower data
>> > > file on open)
>> > > can be done in overlayfs as well.
>> > > (*) currently, overlayfs needs to lookup the lower file also for st_blocks.
>> > >
>> > > I am not saying that it should be done or that Miklos will agree to make
>> > > this change in overlayfs, but that seems to be the major difference.
>> > > getxattr may have some extra cost depending on in-inode xattr format
>> > > of erofs, but specifically, the metacopy getxattr can be avoided if this
>> > > is a special overlayfs RO mount that is marked as EVERYTHING IS
>> > > METACOPY.
>> > >
>> > > I don't expect you guys to now try to hack overlayfs and explore
>> > > this path to completion.
>> > > My expectation is that this information will be clearly visible to anyone
>> > > reviewing future submission, e.g.:
>> > >
>> > > - This is the comparison we ran...
>> > > - This is the reason that composefs gives better results...
>> > > - It MAY be possible to optimize erofs/overlayfs to get to similar results,
>> > >   but we did not try to do that
>> > >
>> > > It is especially important IMO to get the ACK of both Gao and Miklos
>> > > on your analysis, because remember than when this thread started,
>> > > you did not know about the metacopy option and your main argument
>> > > was saving the time it takes to create the overlayfs layer files in the
>> > > filesystem, because you were missing some technical background on overlayfs.
>> >
>> > we knew about metacopy, which we already use in our tools to create
>> > mapped image copies when idmapped mounts are not available, and also
>> > knew about the other new features in overlayfs.  For example, the
>> > "volatile" feature which was mentioned in your
>> > Overlayfs-containers-lpc-2020 talk, was only submitted upstream after
>> > begging Miklos and Vivek for months.  I had a PoC that I used and tested
>> > locally and asked for their help to get it integrated at the file
>> > system layer, using seccomp for the same purpose would have been more
>> > complex and prone to errors when dealing with external bind mounts
>> > containing persistent data.
>> >
>> > The only missing bit, at least from my side, was to consider an image
>> > that contains only overlay metadata as something we could distribute.
>> >
>> 
>> I'm glad that I was able to point this out to you, because now the comparison
>> between the overlayfs and composefs options is more fair.
>> 
>> > I previously mentioned my wish of using it from a user namespace, the
>> > goal seems more challenging with EROFS or any other block devices.  I
>> > don't know about the difficulty of getting overlay metacopy working in a
>> > user namespace, even though it would be helpful for other use cases as
>> > well.
>>
>
> If you decide to try and make this work with overlayfs I can to cut out
> time and help with both review and patches. Because I can see this being
> beneficial for use-cases we have with systemd as well and actually being
> used by us as we do make heavy use of overlayfs already and probably
> will do even more so in the future on top of erofs.
>
> (As a sidenote, in the future, idmapped mounts can be made useable from
> userns and there's a todo and ideas for this on
> https://uapi-group.org/kernel-features.

we won't need metacopy to clone images once idmapped works in a userns,
but I think it is still good to have, if possible.  Not only for the
interesting combination Amir suggested but also for speeding up a bunch
of other operations that currently end up in a complete copy-up.


> Additionally, I want users to have the ability to use them without any
> userns in the mix at all. Not just because there are legitimate users
> that don't need to allocate a userns at all but also because then we can
> do stuff like map down a range of ids to a single id (what probably nfs
> would call "squashing") and other stuff.)