linux-kernel - Re: upcoming kerneloops.org item: get_page_from

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Message-ID: <20090625203743.GD6472@mit.edu>
Date:	Thu, 25 Jun 2009 16:37:43 -0400
From:	Theodore Tso <tytso@....edu>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	penberg@...helsinki.fi, arjan@...radead.org,
	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	npiggin@...e.de, linux-ext4@...r.kernel.org
Subject: Re: upcoming kerneloops.org item: get_page_from_freelist

On Thu, Jun 25, 2009 at 01:18:59PM -0700, David Rientjes wrote:
> Isn't there also a problem in jbd2_journal_write_metadata_buffer(), 
> though?
> 
> 		tmp = jbd2_alloc(bh_in->b_size, GFP_NOFS);
		...
> 		memcpy(tmp, mapped_data + new_offset, jh2bh(jh_in)->b_size);
> 
> jbd2_alloc() is just a wrapper to __get_free_pages() and if it fails, it 
> appears as though the memcpy() would cause a NULL pointer.

Nicely spotted.  Yeah, that's a bug; we need to do something about
that one, too.  And what we're doing is a bit silly; it may make sense
to use __get_free_pages if filesystem blocksize == PAGE_SIZE, but
otherwise we should be using a sub-page allocator.  Right now, we're
chewing up a 16k PPC page for every 4k filesystem metadata page
allocated in journal_write_metadata_buffer(), and on x86, for the
(admittedly uncommon) 1k block filesystem, we'd be chewing up a 4k
page for a 1k block buffer.

Both of these problems exist for both ext3 and ext4.

     	      	       	     	      		- Ted
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