linux-kernel - Re: Commit "new primitive: discard_new

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Message-ID: <20180802023654.t7xujjcv7ugq5ru4@mobile.celeiro.br>
Date:   Wed, 1 Aug 2018 23:36:54 -0300
From:   Rafael David Tinoco <rafael.tinoco@...aro.org>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Commit "new primitive: discard_new_inode()" introduces a problem

On Wed, Aug 01, 2018 at 05:27:39PM +0100, Al Viro wrote:
> On Wed, Aug 01, 2018 at 05:16:48PM +0100, Al Viro wrote:
> > On Wed, Aug 01, 2018 at 05:11:51PM +0100, Al Viro wrote:
> > > On Wed, Aug 01, 2018 at 06:43:56PM +0300, Kirill Tkhai wrote:
> > > > #git status
> > > > HEAD detached at 22dc9a168272
> > > 
> > > Wait a sec - just that commit?  With discard_new_inode() not even
> > > used anywhere?  Then any of those -ENOMEM is disguised ERR_PTR(-ESTALE)
> > > from find_inode()/find_inode_fast(), i.e. an inode found by
> > > iget5_locked() (your /etc is on ext4) with I_CREATING still set.
> > 
> > iget_locked(), that is.
> > 
> > > And that is set only by insert_inode_locked()/insert_inode_locked4(),
> > > and only until unlock_new_inode().
> > > 
> > > Could you slap WARN_ON(1) before these return ERR_PTR(-ESTALE) in
> > > find_inode{,_fast}() and see if that's what's getting triggered?
> 
> ... and I still can't reproduce it here - exact same tree, same .config,
> same reproducer.  The only difference is that it's a gcc-5 build, but
> I can do gcc-6 as well; doubt that it'll change anything.

I bisected an issue with our functional tests to this commit also.

LTP started failing for -next for tests depending on remounts to RO.

Link: https://bugs.linaro.org/show_bug.cgi?id=3957

Reproducer:

    # testfile = ext4 filesystem

    mount ./testfile /mnt
    rm -f /mnt/read-only
    touch /mnt/read-only
    mount -o remount,ro /mnt
    ls /mnt
    $ ls /mnt
    ls: cannot access '/mnt/read-only': Cannot allocate memory

Could not reproduce with xfs.

More information on investigation:

https://bugs.linaro.org/show_bug.cgi?id=3957#c3
https://bugs.linaro.org/show_bug.cgi?id=3957#c4

Trace of a failing acct (thought originally related):

https://bugs.linaro.org/show_bug.cgi?id=3957#c5
do_filp_open() -> path_openat() -> do_last() -> lookup_open()
(check ext4_lookup() and beyond)

I'm testing your patch right now. Will inform soon.

-Rafael