linux-kernel - Re: [PATCH] rd: Use a private inode for backing storage

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Message-Id: <200710211428.55611.nickpiggin@yahoo.com.au>
Date:	Sun, 21 Oct 2007 14:28:55 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	"Theodore Ts'o" <tytso@....edu>, stable@...nel.org
Subject: Re: [PATCH] rd: Use a private inode for backing storage

On Saturday 20 October 2007 08:51, Eric W. Biederman wrote:
> Currently the ramdisk tries to keep the block device page cache pages
> from being marked clean and dropped from memory.  That fails for
> filesystems that use the buffer cache because the buffer cache is not
> an ordinary buffer cache user and depends on the generic block device
> address space operations being used.
>
> To fix all of those associated problems this patch allocates a private
> inode to store the ramdisk pages in.
>
> The result is slightly more memory used for metadata, an extra copying
> when reading or writing directly to the block device, and changing the
> software block size does not loose the contents of the ramdisk.  Most
> of all this ensures we don't loose data during normal use of the
> ramdisk.
>
> I deliberately avoid the cleanup that is now possible because this
> patch is intended to be a bug fix.

This just breaks coherency again like the last patch. That's a
really bad idea especially for stable (even if nothing actually
was to break, we'd likely never know about it anyway).

Christian's patch should go upstream and into stable. For 2.6.25-6,
my rewrite should just replace what's there. Using address spaces
to hold the ramdisk pages just confuses the issue even if they
*aren't* actually wired up to the vfs at all. Saving 20 lines is
not a good reason to use them.

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