linux-ext4 - Re: [PATCH 1/2] jbd2: check bh->b_data for NULL in jbd2_journal_get_descriptor

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Date:	Thu, 6 Jun 2013 11:02:55 +0300
From:	Ruslan Bilovol <ruslan.bilovol@...com>
To:	"Theodore Ts'o" <tytso@....edu>,
	Ruslan Bilovol <ruslan.bilovol@...com>,
	adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] jbd2: check bh->b_data for NULL in
 jbd2_journal_get_descriptor_buffer before memset()

Hi Ted,

On Tue, Jun 4, 2013 at 4:37 PM, Theodore Ts'o <tytso@....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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