Re: Oops while going into hibernate

[Ted Ts'o <tytso@mit.edu>]

On Wed, Jan 12, 2011 at 07:44:17PM -0500, Theodore Tso wrote:
> 
> You said originally that the oops was happening "while going into
> hibernation right after resuming with...".  So that means you did a
> successful suspend/resume, and then the second suspend caused the
> oops?  It looks like somehow the pages were left marked as dirty, so
> the writeback daemons attempted writing back a page to an inode
> which was never opened read/write (and in fact as a text page for
> /usr/bin/killall, was mapped read/only).  Given that ext4
> initializes jinode only when the file is opened read/write, the fact
> that it is null, and the fact that it makes no sense that a program
> would be modifying /usr/bin/killall as part of a suspend/resume, it
> looks very much like we just unmasked a software suspend bug....

... and I think I've found the problem.  In kernel/power/block_io.c,
in the function submit(), we see this:

	if (bio_chain == NULL) {
		submit_bio(bio_rw, bio);
		wait_on_page_locked(page);
		if (rw == READ)
			bio_set_pages_dirty(bio); <====
		bio_put(bio);

So when we read in pages from the software suspend device, we end up
marking the pages as dirty(!).  I'm guessing this was caused by a copy
and paste from the only other caller of bio_set_pages_dirty(), which
is the direct I/O code, which needs this when we are writing from a
file into a user-provided buffer.  But for restoring from a software
suspend case, this is as far as I can tell wholely inappropriate.

This causes needless writes, which is bad even before ext4 unmasked
the problem.  I will send a patch under separate cover; could you give
it a try and see if it fixes your crash?

I will look into bulletproofing ext4 by adding checks for this case
and printing warning messages, but neverthe less, I think the root
cause is actually in the hibernation's bio code.

					- Ted