[Patch] Catch filesystems lacking s_bdi

[Jens Axboe]

On Mon, Apr 26 2010, Jörn Engel wrote:
> > Yeah, it's a bad situation to be in. I changed that BUG_ON() to a
> > WARN_ON(). I'm not too worried about that part, I'm more worried about
> > the file system changed. OTOH, they do lack proper flushing now, so it's
> > likely not a huge risk from that perspective.
> 
> It is worse still.  Using mtd->backing_dev_info results in 
> kernel BUG at fs/fs-writeback.c:157
> 
> which is BUG_ON(!work->seen); in bdi_queue_work().  Logfs is affected
> and I bet jffs2 is as well.  So much for dwmw2 or me actually testing
> the fix. :(
> 
> I did a hexdump to see what sb->s_bdi actually contained and the result
> was this:
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00                          ........
> 
> Which should be mtd_bdi_unmappable.  And at this point I have to admit
> being clueless.  What exactly should a struct backing_dev_info contain
> and for what purposes?  And where is this documented?

The important bit is that each bdi must be initialized and registered if
it's going to be handling dirty data, it can't just be a static
placeholder. See the bdi_setup_and_register() helper I added.

-- 
Jens Axboe