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

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Message-ID: <465A6078.6010804@yahoo.com.au>
Date:	Mon, 28 May 2007 14:54:16 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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

Eric W. Biederman wrote:
> Nick Piggin <nickpiggin@...oo.com.au> writes:
> 
> 
>>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 state 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.
>>
>>Ouch indeed!
>>
>>But can we ever have a dirty page at init_page_buffers-time?
> 
> 
> Definitely, and it was a royal pain to trace the bug that this
> caused.  An initial ramdisk having pieces disappear after mkfs
> is called can look like the entire machine is dying.
> 
> 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.


>>I would have thought we can fix this simply by removing the
>>broken ramdisk_set_page_dirty (as far as the comment goes, we
>>set CAP_NO_ACCT_DIRTY anyway, so the normal set_page_dirty
>>should handle everything properly, no?).
> 
> 
> No.  I don't know where accounting comes into play.  I didn't
> trace that path.  But if we have a non-dirty ramdisk page with
> buffers (basically a hole in the middle or at the end of the ramdisk).
> We need to set the buffer dirty bits when we write to it.

Accounting is done in set_page_dirty.

> 
> So I don't see how it would make sense to reuse the generic
> set_page_dirty, and handling all of the logic in set_page_dirty
> to dirty the buffer heads seemed to have made the most sense.

That's what the generic set_page_dirty does. What I want to know
is why *doesn't* it make sense to reuse the generic set_page_dirty?
Unless there is a good reason, then reusing is better than writing
your own.

-- 
SUSE Labs, Novell Inc.
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