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    SubjectRe: [Patch] Catch filesystems lacking s_bdi
    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

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