Date:	Thu, 16 Nov 2006 13:27:24 -0800
From:	Andrew Morton <akpm@...l.org>
To:	cmm@...ibm.com
Cc:	Hugh Dickins <hugh@...itas.com>, Mel Gorman <mel@...net.ie>,
	"Martin J. Bligh" <mbligh@...igh.org>,
	linux-kernel@...r.kernel.org,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Boot failure with ext2 and initrds

On Thu, 16 Nov 2006 12:15:16 -0800
Mingming Cao <cmm@...ibm.com> wrote:

> On Thu, 2006-11-16 at 01:13 -0800, Andrew Morton wrote:
> > On Thu, 16 Nov 2006 00:49:20 -0800
> > Mingming Cao <cmm@...ibm.com> wrote:
> > 
> > > On Wed, 2006-11-15 at 23:22 -0800, Andrew Morton wrote:
> > > > On Wed, 15 Nov 2006 22:55:43 -0800
> > > > Mingming Cao <cmm@...ibm.com> wrote:
> > > > 
> > > > > Hmm, maxblocks, in bitmap_search_next_usable_block(),  is the end block 
> > > > > number of the range  to search, not the lengh of the range. maxblocks 
> > > > > get passed to ext2_find_next_zero_bit(), where it expecting to take the 
> > > > > _size_ of the range to search instead...
> > > > > 
> > > > > Something like this: (this is not a patch)
> > > > >   @@ -524,7 +524,7 @@ bitmap_search_next_usable_block(ext2_grp
> > > > >    	ext2_grpblk_t next;
> > > > > 
> > > > >    -  	next = ext2_find_next_zero_bit(bh->b_data, maxblocks, start);
> > > > >    +  	next = ext2_find_next_zero_bit(bh->b_data, maxblocks-start + 1, start);
> > > > > 	if (next >= maxblocks)
> > > > >    		return -1;
> > > > >    	return next;
> > > > >    }
> > > > 
> > > > yes, the `size' arg to find_next_zero_bit() represents the number of bits
> > > > to scan at `offset'.
> > > > 
> > > > So I think your change is correctish.  But we don't want the "+ 1", do we?
> > > > 
> > > I think we still need the "+1", maxblocks here is the ending block of
> > > the reservation window, so the number of bits to scan =end-start+1.
> > > 
> > > > If we're right then this bug could cause the code to scan off the end of the
> > > > bitmap.  But it won't explain Hugh's bug, because of the if (next >= maxblocks).
> > > > 
> > > 
> > > Yeah.. at first I thought it might be related, then, thinked it over,
> > > the bug only makes the bits to scan larger, so if find_next_zero_bit()
> > > returns something off the end of bitmap, that is fine, it just
> > > indicating that there is no free bit left in the rest of bitmap, which
> > > is expected behavior. So bitmap_search_next_usable_block() fail is the
> > > expected. It will move on to next block group and try to create a new
> > > reservation window there.
> > 
> > I wonder why it's never oopsed.  Perhaps there's always a zero in there for
> > some reason.
> > 
> 
> Why you think it should oopsed?  Even if find_next_zero_bit() finds a
> zero bit beyond of the end of bitmap, the check next > maxblocks will
> catch this and make sure we are not taking a zero bit out of the bitmap
> range, so it fails as expected.

If it can read off the end of the buffer, it can oops.  With
CONFIG_DEBUG_PAGEALLOC, especially.


> > > That does not explain the repeated reservation window add and remove
> > > behavior Huge has reported. 
> > 
> > I spent quite some time comparing with ext3.  I'm a bit stumped and I'm
> > suspecting that the simplistic porting the code is now OK, but something's
> > just wrong.
> > 
> > I assume that the while (1) loop in ext3_try_to_allocate_with_rsv() has
> > gone infinite.  I don't see why, but more staring is needed.
> > 
> 
> The loop should not go forever, it will stops when there is no window
> with free bit to reserve in the given block group.

It seems to have done so in Hugh's testing, but there's some question there
now.  Although I didn't check to see if there's a significant difference
between Hugh's patch and mine.


> > What lock protects the fields in struct ext[234]_reserve_window from being
> > concurrently modified by two CPUs?  None, it seems.  Ditto
> > ext[234]_reserve_window_node.  i_mutex will cover it for write(), but not
> > for pageout over a file hole.  If we end up with a zero- or negative-sized
> > window then odd things might happen.
> > 
> 
> Yes, trucate_mutex protect both struct ext[234]_reserve_window and ext
> [234]_reserve_window_node, and struct ext[234]_block_alloc_info.
> Actually I think truncate_mutex protects all data structures related to
> block allocation/mapping structures.

Yes.  I guess ext2 needs a new mutex for this.  Sad.
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