linux-kernel - Re: Question on fscrypt_d_revalidate() and fstest generic/429

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Date:   Tue, 16 May 2017 15:07:09 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Richard Weinberger <richard@....at>
Cc:     linux-fscrypt@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Theodore Ts'o <tytso@....edu>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Gstir <david@...ma-star.at>,
        David Oberhollenzer <david.oberhollenzer@...ma-star.at>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        Artem Bityutskiy <dedekind1@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: Question on fscrypt_d_revalidate() and fstest generic/429

On Tue, May 16, 2017 at 08:47:43AM +0200, Richard Weinberger wrote:
> Eric,
> 
> Am 16.05.2017 um 01:25 schrieb Eric Biggers:
> > On Mon, May 15, 2017 at 09:51:03PM +0200, Richard Weinberger wrote:
> >>>
> >>> The test is repeatedly creating and removing a directory "dir" while lookups are
> >>> being done in it.  It seems the problem is that many dentries are being created
> >>> for "dir", and they pin many different inodes, all at the same time.  This
> >>> actually happens for ext4 too; it just doesn't cause an observable error.
> >>>
> >>> I doubt it's the right solution to make fscrypt_d_revalidate() look at
> >>> ->i_nlink, since ->d_revalidate() is meant to validate the filename, not the
> >>> inode.  I think there is probably a VFS bug that is causing the dentries to not
> >>> be freed.
> >>
> >> Not sure. Al? :-)
> >>
> > 
> > I can reproduce this on an unencrypted directory after updating path_init() in
> > fs/namei.c to always clear LOOKUP_RCU, so that all path lookups are done in
> > ref-walk mode.  So I think fscrypt_d_revalidate() was only relevant because it
> > causes all path lookups to drop out of rcu-walk mode.
> 
> On ext4 or UBIFS?
> 
> Thanks,
> //richard

Both; the inode "leak" isn't filesystem-specific, beyond the fact that UBIFS
apparently limits the number of inodes on its orphan list while ext4 does not.
(How do I know it happens on ext4 then?  /proc/slabinfo shows that lots of ext4
inodes have been allocated, and after an unclean shutdown and remounting, it's
reported that 50,000+ orphan inodes were deleted.)

There are lots of cases in which path lookups switch from rcu-walk mode into
ref-walk mode, so the fact that it was being caused by ->d_revalidate() in this
specific situation isn't really important, IMO.

I think the actual fix would be something along the lines of making vfs_rmdir()
unhash any negative child dentries, so that they get "killed" by dput() later.

Eric