linux-ext4 - Re: [PATCH] ext4: drop file_update_time from ext4_dax

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Date:	Tue, 24 Nov 2015 06:55:34 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Ted Ts'o <tytso@....edu>, linux-nvdimm@...ts.01.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jan Kara <jack@...e.com>
Subject: Re: [PATCH] ext4: drop file_update_time from ext4_dax_fault

On Mon, Nov 23, 2015 at 03:15:56PM +0100, Jan Kara wrote:
> On Fri 20-11-15 14:03:44, Dave Chinner wrote:
> > On Thu, Nov 19, 2015 at 04:18:13PM -0800, Dan Williams wrote:
> > >  WARNING: CPU: 0 PID: 2133 at fs/fs-writeback.c:2065 __mark_inode_dirty+0x261/0x350()
> > >  bdi-block not registered
> >    ^^^^^^^^^^^^^^^^^^^^^^^^
> > >  [..]
> > >  Call Trace:
> > >   [<ffffffff81459f62>] dump_stack+0x44/0x62
> > >   [<ffffffff810a2052>] warn_slowpath_common+0x82/0xc0
> > >   [<ffffffff810a20ec>] warn_slowpath_fmt+0x5c/0x80
> > >   [<ffffffff812831a1>] __mark_inode_dirty+0x261/0x350
> > >   [<ffffffff8126d109>] generic_update_time+0x79/0xd0
> > >   [<ffffffff8126d28d>] file_update_time+0xbd/0x110
> > >   [<ffffffff812e4bc8>] ext4_dax_fault+0x68/0x110
> > >   [<ffffffff811f816e>] __do_fault+0x4e/0xf0
> > >   [<ffffffff811fc2a7>] handle_mm_fault+0x5e7/0x1b50
> > >   [<ffffffff811fbd11>] ? handle_mm_fault+0x51/0x1b50
> > >   [<ffffffff810689c1>] __do_page_fault+0x191/0x3f0
> > >   [<ffffffff81068cef>] trace_do_page_fault+0x4f/0x120
> > >   [<ffffffff8106314a>] do_async_page_fault+0x1a/0xa0
> > >   [<ffffffff81902678>] async_page_fault+0x28/0x30
> > 
> > Doesn't this indicate some problem at the block/bdi level?
> > __mark_inode_dirty() should not throw warnings like this regardless
> > of where it is called from...
> 
> The warning warns that we are dirtying inodes against a backing device that
> advertises it does writeback but is not registered with writeback code (so
> flusher does not run for it). That makes sense but people seem to be
> experimenting a lot with removing devices under filesystems lately and so
> this warning triggers. Probably we have to refine it to not trigger for
> such cases...

Or, alternatively, have the block devices inform the filesystem that
it no longer has a valid bdi backing it. That way the filesystem can
error out at a higher layer and we never get to the point of trying
to mark an inode dirty when the backing device has been unplugged...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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