linux-ext4 - RE: Can ext4_break_layouts() ever fail?

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Message-ID: <2807E5FD2F6FDA4886F6618EAC48510E79D2A98C@CRSMSX101.amr.corp.intel.com>
Date:   Mon, 20 May 2019 16:24:27 +0000
From:   "Weiny, Ira" <ira.weiny@...el.com>
To:     Jan Kara <jack@...e.cz>
CC:     Theodore Ts'o <tytso@....edu>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: RE: Can ext4_break_layouts() ever fail?

> On Fri 17-05-19 13:17:47, Ira Weiny wrote:
> > On Fri, May 17, 2019 at 11:02:52AM +0200, Jan Kara wrote:
> > > On Thu 16-05-19 13:56:15, Ira Weiny wrote:
> > >
> > > > It looks to me like it is possible for ext4_break_layouts() to
> > > > fail if
> > > > prepare_to_wait_event() sees a pending signal.  Therefore I think
> > > > this is a bug in ext4 regardless of how I may implement a truncate
> failure.
> > >
> > > Yes, it's a bug in ext4.
> > >
> > > > --- a/fs/ext4/inode.c
> > > > +++ b/fs/ext4/inode.c
> > > > @@ -5648,6 +5648,8 @@ int ext4_setattr(struct dentry *dentry,
> struct iattr *attr)
> > > >                 if (rc) {
> > > >                         up_write(&EXT4_I(inode)->i_mmap_sem);
> > > >                         error = rc;
> > > > +                       if (orphan)
> > > > +                               ext4_orphan_del(NULL, inode);
> > >
> > > This isn't quite correct. This would silence the warning but leave
> > > the inode in on-disk orphan list. That is OK in case of fs-meltdown
> > > types of failures like IO errors for metadata, aborted journal, or stuff like
> that.
> > > But failing ext4_break_layouts() needs to be handled gracefully
> > > maintaining fs consistency. So you rather need something like:
> > >
> > > 			if (orphan && inode->i_nlink > 0) {
> > > 				handle_t *handle;
> > >
> > > 				handle = ext4_journal_start(inode,
> > > 						EXT4_HT_INODE, 3);
> > > 				if (IS_ERR(handle)) {
> > > 					ext4_orphan_del(NULL, inode);
> > > 					goto err_out;
> > > 				}
> > > 				ext4_orphan_del(handle, inode);
> > > 				ext4_journal_stop(handle);
> > > 			}
> > >
> >
> > Thanks!  Unfortunately, even with your suggestion something is still
> > wrong with my code.
> >
> > For some reason this does not seem to be "canceling" the truncate
> > completely.  With my test code for FS DAX which fails
> > ext4_break_layout() the file is being truncated and an application
> > which is writing past that truncation is getting a SIGBUS.
> 
> Looking at the code again, I'm not really surprised. The path bailing out of
> truncate in case ext4_break_layouts() fails is really hosed. The problem is
> that when we get to ext4_break_layouts(), we have already updated i_size
> and i_disksize and we happily leave them at their new values when bailing
> out. So we need to somewhat reorder the stuff we do in ext4_setattr(). I'll
> send a patch for that since it needs some considerations for proper lock
> ordering etc... Thanks for experimenting with this :)
> 

I should have sent something last night but yes I came to the same conclusion through some simple experiments.

I agree that the locking and other considerations would trip me up.  So I'm not opposed to you helping here.  I had more than 1 problem with either crashes or hangs while playing with the code.  :-/

Thanks,
Ira

> 								Honza
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR