linux-kernel - Re: [PATCH 0/7] overlay filesystem: request for inclusion

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Date:	Fri, 15 Jul 2011 15:07:47 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"J. R. Okajima" <hooanon05@...oo.co.jp>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	NeilBrown <neilb@...e.de>, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, apw@...onical.com, nbd@...nwrt.org,
	hramrach@...trum.cz, jordipujolp@...il.com, ezk@....cs.sunysb.edu
Subject: Re: [PATCH 0/7] overlay filesystem: request for inclusion

"J. R. Okajima" <hooanon05@...oo.co.jp> writes:

> Miklos Szeredi:
>> st_nlink==1 for directories is widely accepted way of saying that the
>> number of subdirectories is unknown.  Various filesystems already do
>> this, and versions of GNU utils that I have come across accept it.
>
> When the upperdir is tmpfs, the link count of the directories in it will
> not be 1, won't it?

In overlayfs directory will have a link count of one if it's a "merged"
directory, regardless of the filesystem types used.  If it's not a
merged directory, then st_nlink will correctly refrect the number of
subdirs.

>> I hadn't thought about this case, so overlayfs would modify the lower
>> filesystem in that case, which is a no-no.
>>
>> Following patch fixes this and return -EROFS for the above case.  I also
>> updated the non-standard section in the docs.
>
> Hmm, such changes to mnt_flags looks slightly rude to me. Do we have to
> consider about these?
> - there may exist files opened as RW on the lower.
> - when overlayfs is unmounted, it should restore the original mnt_flags.
> - (there may exist more...)
>
> If overlayfs doesn't expect the lower mounted as RW, then it might be
> better to reject it simply at mounting.

Overlayfs will create a private clone of both the lower and upper
mounts.  The mnt_flags are only modified on the private clone, whose
sole user is overlayfs itself.

Thanks,
Miklos
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