[Patch] Catch filesystems lacking s_bdi

[Jens Axboe]

On Mon, Apr 26 2010, Jens Axboe wrote:
> 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.

Took a quick look, and you want bdi_setup_and_register() for the three
bdis listed in mtdbdi.c in mtdcore.c:init_mtd(). Or manual bdi_init()
and bdi_register(). I'll take a look post-dinner. Either is workable,
but since the flags are already setup, the latter may be cleaner.

-- 
Jens Axboe