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

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Date:	Fri, 3 Dec 2010 13:15:20 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	Mike Snitzer <snitzer@...hat.com>,
	LVM general discussion and development 
	<linux-lvm@...hat.com>, Spelic <spelic@...ftmail.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	xfs@....sgi.com, dm-devel@...hat.com
Subject: Re: Bugs in mkfs.xfs, device mapper, xfs, and /dev/ram

On Sat, Dec 04, 2010 at 04:11:40AM +1100, Nick Piggin wrote:
> > 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.

How many programs actually depend on BLKFLSBUF dropping the pages used
in /dev/ram?  The fact that it did this at all was a historical
accident of how the original /dev/ram was implemented (in the buffer
cache directly), and not anything that was intended.  I think that's
something that we should be able to fix, since the number of programs
that knowly operate on the ramdisk is quite small.  Just a few system
programs used by distributions in their early boot scripts....

So I would argue for dropping the "special" behavior of BLKFLSBUF for
/dev/ram.

							- Ted
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