Re: upcoming kerneloops.org item: get_page_from_freelist

[Theodore Tso <tytso@mit.edu>]

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