From: Al Viro <viro@ZenIV.linux.org.uk>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: John Ogness <john.ogness@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Christoph Hellwig <hch@lst.de>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fs: Teach path_connected to handle nfs filesystems with multiple roots.
Date: Thu, 15 Mar 2018 22:34:45 +0000 [thread overview]
Message-ID: <20180315223444.GA30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <87muzai6rm.fsf_-_@xmission.com>
On Wed, Mar 14, 2018 at 06:20:29PM -0500, Eric W. Biederman wrote:
>
> On nfsv2 and nfsv3 the nfs server can export subsets of the same
> filesystem and report the same filesystem identifier, so that the nfs
> client can know they are the same filesystem. The subsets can be from
> disjoint directory trees. The nfsv2 and nfsv3 filesystems provides no
> way to find the common root of all directory trees exported form the
> server with the same filesystem identifier.
>
> The practical result is that in struct super s_root for nfs s_root is
> not necessarily the root of the filesystem. The nfs mount code sets
> s_root to the root of the first subset of the nfs filesystem that the
> kernel mounts.
>
> This effects the dcache invalidation code in generic_shutdown_super
> currently called shrunk_dcache_for_umount and that code for years
> has gone through an additional list of dentries that might be dentry
> trees that need to be freed to accomodate nfs.
>
> When I wrote path_connected I did not realize nfs was so special, and
> it's hueristic for avoiding calling is_subdir can fail.
>
> The practical case where this fails is when there is a move of a
> directory from the subtree exposed by one nfs mount to the subtree
> exposed by another nfs mount. This move can happen either locally or
> remotely. With the remote case requiring that the move directory be cached
> before the move and that after the move someone walks the path
> to where the move directory now exists and in so doing causes the
> already cached directory to be moved in the dcache through the magic
> of d_splice_alias.
>
> If someone whose working directory is in the move directory or a
> subdirectory and now starts calling .. from the initial mount of nfs
> (where s_root == mnt_root), then path_connected as a heuristic will
> not bother with the is_subdir check. As s_root really is not the root
> of the nfs filesystem this heuristic is wrong, and the path may
> actually not be connected and path_connected can fail.
>
> The is_subdir function might be cheap enough that we can call it
> unconditionally. Verifying that will take some benchmarking and
> the result may not be the same on all kernels this fix needs
> to be backported to. So I am avoiding that for now.
>
> Filesystems with snapshots such as nilfs and btrfs do something
> similar. But as the directory tree of the snapshots are disjoint
> from one another and from the main directory tree rename won't move
> things between them and this problem will not occur.
>
> Cc: stable@vger.kernel.org
> Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
> Fixes: 397d425dc26d ("vfs: Test for and handle paths that are unreachable from their mnt_root")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>
> Al do you want to push this one to Linus or shall I?
Applied; I think there might be a helper lurking in there, but for now
that'll do.
next prev parent reply other threads:[~2018-03-15 22:34 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-22 23:50 [PATCH v2 0/6] fs/dcache: avoid trylock loops John Ogness
2018-02-22 23:50 ` [PATCH v2 1/6] fs/dcache: Remove stale comment from dentry_kill() John Ogness
2018-02-22 23:50 ` [PATCH v2 2/6] fs/dcache: Move dentry_kill() below lock_parent() John Ogness
2018-02-22 23:50 ` [PATCH v2 3/6] fs/dcache: Avoid the try_lock loop in d_delete() John Ogness
2018-02-23 2:08 ` Al Viro
2018-02-22 23:50 ` [PATCH v2 4/6] fs/dcache: Avoid the try_lock loops in dentry_kill() John Ogness
2018-02-23 2:22 ` Al Viro
2018-02-23 3:12 ` Al Viro
2018-02-23 3:16 ` Al Viro
2018-02-23 5:46 ` Al Viro
2018-02-22 23:50 ` [PATCH v2 5/6] fs/dcache: Avoid a try_lock loop in shrink_dentry_list() John Ogness
2018-02-23 3:48 ` Al Viro
2018-02-22 23:50 ` [PATCH v2 6/6] fs/dcache: Avoid remaining " John Ogness
2018-02-23 3:58 ` Al Viro
2018-02-23 4:08 ` Al Viro
2018-02-23 13:57 ` John Ogness
2018-02-23 15:09 ` Al Viro
2018-02-23 17:42 ` Al Viro
2018-02-23 20:13 ` [BUG] lock_parent() breakage when used from shrink_dentry_list() (was Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list()) Al Viro
2018-02-23 21:35 ` Linus Torvalds
2018-02-24 0:22 ` Al Viro
2018-02-25 7:40 ` Al Viro
2018-02-27 5:16 ` dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()) John Ogness
2018-03-12 19:13 ` Al Viro
2018-03-12 20:05 ` Al Viro
2018-03-12 20:33 ` Al Viro
2018-03-13 1:12 ` NeilBrown
2018-04-28 0:10 ` Al Viro
2018-03-12 20:23 ` Eric W. Biederman
2018-03-12 20:39 ` Al Viro
2018-03-12 23:28 ` Eric W. Biederman
2018-03-12 23:52 ` Eric W. Biederman
2018-03-13 0:37 ` Al Viro
2018-03-13 0:50 ` Al Viro
2018-03-13 4:02 ` Eric W. Biederman
2018-03-14 23:20 ` [PATCH] fs: Teach path_connected to handle nfs filesystems with multiple roots Eric W. Biederman
2018-03-15 22:34 ` Al Viro [this message]
2018-03-13 0:36 ` dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()) Al Viro
2018-03-12 22:14 ` Thomas Gleixner
2018-03-13 20:46 ` John Ogness
2018-03-13 21:05 ` John Ogness
2018-03-13 23:59 ` Al Viro
2018-03-14 2:58 ` Matthew Wilcox
2018-03-14 8:18 ` John Ogness
2018-03-02 9:04 ` [BUG] lock_parent() breakage when used from shrink_dentry_list() (was Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list()) Sebastian Andrzej Siewior
2018-02-23 0:59 ` [PATCH v2 0/6] fs/dcache: avoid trylock loops Linus Torvalds
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=20180315223444.GA30522@ZenIV.linux.org.uk \
--to=viro@zeniv.linux.org.uk \
--cc=bigeasy@linutronix.de \
--cc=ebiederm@xmission.com \
--cc=hch@lst.de \
--cc=john.ogness@linutronix.de \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
/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).