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

    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.



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