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SubjectRe: [PATCH 1/2] jbd2: check bh->b_data for NULL in jbd2_journal_get_descriptor_buffer before memset()
From
Hi Ted,

On Tue, Jun 4, 2013 at 4:37 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> 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);
> WARN_ON(1);
> }
> if (bh)
> bh_lru_install(bh);
> } else {
> if (bh->b_data == NULL) {
> pr_crit("b_data NULL after lookup_bh_lru\n");
> WARN_ON(1);
> }
> }
> if (bh)
> touch_buffer(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
> down?
>
> 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.

Yes agree, that's what I told about in my cover letter fir this patch series.
I will debug it with code you mentioned, but the issue appears
very rarely, so I need at lease few days for catching this..

Regards,
Ruslan

>
> Regards,
>
> - Ted
> --
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--
Best regards,
Ruslan Bilvol


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