Re: Bugs in mkfs.xfs, device mapper, xfs, and /dev/ram

Mike Snitzer
2010-12-02T22:06:35Z

On Thu, Dec 02 2010 at 4:22pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Thu, Dec 02 2010 at 9:17am -0500,
> Christoph Hellwig <hch@infradead.org> wrote:
>
> > On Thu, Dec 02, 2010 at 03:14:28PM +0100, Spelic wrote:
> > > On 12/02/2010 03:11 PM, Christoph Hellwig wrote:
> > > >I'm pretty sure you have CONFIG_DEBUG_BLOCK_EXT_DEVT enabled. This
> > > >option must never be enabled, as it causes block devices to be
> > > >randomly renumered. Together with the ramdisk driver overloading
> > > >the BLKFLSBUF ioctl to discard all data it guarantees you to get
> > > >data loss like yours.
> > >
> > > Nope...
> > >
> > > # CONFIG_DEBUG_BLOCK_EXT_DEVT is not set
> >
> > Hmm, I suspect dm-linear's dumb forwarding of ioctls has the same
> > effect.
>
> For the benefit of others:
> - mkfs.xfs will avoid sending BLKFLSBUF to any device whose major is
> ramdisk's major, this dates back to 2004:
> http://oss.sgi.com/archives/xfs/2004-08/msg00463.html
> - but because a kpartx partition overlay (linear DM mapping) is used for
> the /dev/ram0p1 device, mkfs.xfs only sees a device with DM's major
> - so mkfs.xfs sends BLKFLSBUF to the DM device blissfully unaware that
> the backing device (behind the DM linear target) is a brd device
> - DM will forward the BLKFLSBUF ioctl to brd, which triggers
> drivers/block/brd.c:brd_ioctl (nuking the entire ramdisk in the
> process)
>
> So coming full circle this is what hch was referring to when he
> mentioned:
> 1) "ramdisk driver overloading the BLKFLSBUF ioctl ..."
> 2) "dm-linear's dumb forwarding of ioctls ..."
>
> I really can't see DM adding a specific check for ramdisk's major when
> forwarding the BLKFLSBUF ioctl.
>
> brd has direct partition support (see commit d7853d1f8932c) so maybe
> kpartx should just blacklist /dev/ram devices?
>
> Alternatively, what about switching brd away from overloading BLKFLSBUF
> to a real implementation of (overloaded) BLKDISCARD support in brd.c?
> One that doesn't blindly nuke the entire device but that properly
> processes the discard request.

Hmm, any chance we could revisit this approach?