Re: [patch] brd: fix ramdisk regression

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 20 May 2008 22:24:01 +0200
Marcin Krol <hawk@pld-linux.org> wrote:

> >> --- linux-2.6.25/drivers/block/brd.c.orig	2008-04-17 04:49:44.000000000 +0200
> >> +++ linux-2.6.25/drivers/block/brd.c	2008-05-18 01:18:28.381903343 +0200
> >> @@ -442,6 +442,7 @@
> >>  	disk->fops		= &brd_fops;
> >>  	disk->private_data	= brd;
> >>  	disk->queue		= brd->brd_queue;
> >> +	disk->flags |= GENHD_FL_SUPPRESS_PARTITION_INFO;
> >>  	sprintf(disk->disk_name, "ram%d", i);
> >>  	set_capacity(disk, rd_size * 2);
> >>  
> > 
> > Why is it a "regression"?
> > 
> > The change in 2.6.25 was a back-compatible one.
> > 
> > This change is not a back-compatible one and if we're going to now
> > withdraw the newly-added 2.6.25 feature then we should also withdraw it
> > from 2.6.26.x and 2.6.25.x (if that is still under maintenance).  To
> > reduce the incidence of "hey where did my feature go" problems.
> > 
> > Really, life would be simpler if we just left the accidentally-added
> > feature in place.  What problems does it cause?
> 
> All kernels prior to 2.6.25 weren't displaying ramdisks in 
> /proc/partitions. Since there are many userspace tools using infromation 
> from /proc/partitions some of them may now behave incorrectly (I didn't 
> tested any though). For example before 2.6.25 /proc/partitions was empty 
> if no block devices like hard disks and such were detected by kernel. 
> Now all 16 ramdisks are always visible there. Some software may rely on 
> such information (I mean, on empty /proc/partitions).
> 
> There was quite similar situation back in 2004, and ramdisks were 
> excluded back from displaying. Thats why I called this a regression 
> (maybe a bit unfortunate). See this patch for info: 
> http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.3-rc2/2.6.3-rc2-mm1/broken-out/nbd-proc-partitions-fix.patch
> 
> I also think that someone somewhere (long time ago) excluded ramdisks 
> from /proc/partitions for good reasons. It is possible that now such new 
> "feature" is harmless, but I think there are more chances that someone 
> will say "hey, /proc/partitions has changed, now my software doesn't 
> work" then "hey where did my new 2.6.25 feature go". nbd devices are 
> also excluded, maybe for very same (unknown to me) reasons.

I added the above info to the changelog.  But it's still a bit vague,
and as far as I know we don't actually know about any breakage (do we?)

> For me this change isn't problematic. Yes, it did broken my software and 
> yes, I could fix my software but since its working only on specific 
> kernels (precompiled by me) I've chosen to revert kernel change instead 
> to get old behaviour.

Ah, it only affects 2.6.25.x and no earlier -stable kernels.  Not so bad.

Greg, Chris?  Thoughts?