On Thu, Dec 02, 2010 at 04:22:27PM -0500, Mike Snitzer wrote: > On Thu, Dec 02 2010 at 9:17am -0500, > Christoph Hellwig <hch@xxxxxxxxxxxxx> 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. Yeah the situation really sucks (mkfs.jfs doesn't work on ramdisk for the same reason). I want to unfortunately keep ioctl for compatibility, but adding new saner ones would be welcome. Also, having a non-default config or load time parameter for brd, to skip the special case, if that would help testing on older userspace. DISCARD is actually a problem for rd. To actually get proper correctness, you need to preload brd with pages, otherwise when doing stress tests, IO can require memory allocations and deadlock. If we add a discard that frees pages, that introduces the same problem. If you find any option useful for testing, however, patches are fine -- brd pretty much is only useful for testing nowadays. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
