Re: [PATCH 1/2] jbd2: check bh->b_data for NULL injbd2_journal_get_descriptor_buffer before memset()

From: Theodore Ts'o
Date: Tue Jun 04 2013 - 09:38:05 EST

On Tue, Jun 04, 2013 at 02:15:57PM +0300, Ruslan Bilovol wrote:
> > Have you actually seen a case where bh is non-NULL, but bh->b_data is
> > NULL? If not, it might be better to do something like this:
> Yes, this is exactly the situation I observe (bh is non-NULL, but
> bh->b_data is NULL)

Hmm... so the stack trace you sent in the commit description was one
where bh->b_data was NULL? I'm trying to make sure there isn't
something else going on that we don't understand.

Could you put some instrumentation in __find_get_block()? Something like this:

struct buffer_head *
__find_get_block(struct block_device *bdev, sector_t block, unsigned size)
struct buffer_head *bh = lookup_bh_lru(bdev, block, size);

if (bh == NULL) {
bh = __find_get_block_slow(bdev, block);
if (bh->b_data == NULL) {
pr_crit("b_data NULL after find_get_block_slow\n);
if (bh)
} else {
if (bh->b_data == NULL) {
pr_crit("b_data NULL after lookup_bh_lru\n");
if (bh)
return bh;

... and then send me the stack trace after running your reproduction
case. If it turns out the problem is in __find_get_block_slow(),
could you put in similar debugging checks there and try to track it

I'm pretty sure the case of bh non-NULL and bh->b_data NULL is never
supposed to happen, and while we could just put a check where you
suggested, there are plenty of other places which use __getblk(), and
there may be other bugs that are hiding here.


- Ted
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