linux-kernel - Re: [PATCH 1/3] Preserve the dirty bit in init_page

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Message-ID: <m1veed82mz.fsf@ebiederm.dsl.xmission.com>
Date:	Mon, 28 May 2007 08:05:24 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] Preserve the dirty bit in init_page_buffers

Nick Piggin <nickpiggin@...oo.com.au> writes:

> Nick Piggin wrote:
>> Eric W. Biederman wrote:
>
>>> When we initialize the ramdisk by writing to /dev/ram0 usually in
>>> init/do_mounts_rd.c we don't allocate buffer heads but we do set
>>> the dirty bit, and the page is in the page cache.  So when we
>>> later call getblk it reuses the same page and then calls
>>> init_page_buffers.
>>
>>
>> Hmm, so this would be a problem for block_dev.c as well, then?
>> Because it would be possible to have a dirty block dev page
>> have its buffers reclaimed and then reinitialised via
>> init_page_buffers, AFAIKS.
>
> Oh, no, try_to_free_buffers won't drop dirty buffers. However we
> could still set_page_dirty of a block device page without buffers
> via an mmap.

After the page is made dirty via mmap we have:
sys_write -> ... -> block_prepare_write -> ... -> create_empty_buffers.

I suspect that is a pretty rare case but it does indeed seem to exist
as a problem.

Eric
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