linux-kernel - Re: [PATCH] rd: Preserve the dirty bit in init_page

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Message-ID: <m1przfh06r.fsf@ebiederm.dsl.xmission.com>
Date:	Tue, 16 Oct 2007 03:35:08 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] rd: Preserve the dirty bit in init_page_buffers()

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

> On Tuesday 16 October 2007 08:40, Eric W. Biederman wrote:
>> The problem:  When we are trying to free buffers try_to_free_buffers()
>> will look at ramdisk pages with clean buffer heads and remove the
>> dirty bit from the page.  Resulting in ramdisk pages with data that get
>> removed from the page cache.  Ouch!
>>
>> Buffer heads appear on ramdisk pages when a filesystem calls
>> __getblk() which through a series of function calls eventually calls
>> init_page_buffers().
>>
>> So to fix the mismatch between buffer head and page state this patch
>> modifies init_page_buffers() to transfer the dirty bit from the page to
>> the buffer heads like we currently do for the uptodate bit.
>>
>> This patch is safe as only __getblk calls init_page_buffers, and
>> there are only two implementations of block devices cached in the
>> page cache.  The generic implementation in block_dev.c and the
>> implementation in rd.c.
>>
>> The generic implementation of block devices always does everything
>> in terms of buffer heads so it always has buffer heads allocated
>> before a page is marked dirty so this change does not affect it.
>
> This is probably a good idea. Was this causing the reiserfs problems?
> If so, I think we should be concentrating on what the real problem
> is with reiserfs... (or at least why this so obviously correct
> looking patch is wrong).

I think it was my cleanup patch that was sitting on top of this,
That caused the problems.

Eric
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