linux-kernel - Re: [PATCH] add check do_direct

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Date:	Mon, 30 Jul 2007 17:15:23 -0700
From:	Badari Pulavarty <pbadari@...ibm.com>
To:	Zach Brown <zach.brown@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Jin <joe.jin@...cle.com>,
	gurudas pai <gurudas.pai@...cle.com>,
	torvalds@...ux-foundation.org, jens.axboe@...cle.com,
	lkml <linux-kernel@...r.kernel.org>, wen.gang.wang@...cle.com
Subject: Re: [PATCH] add check do_direct_IO() return val

On Mon, 2007-07-30 at 16:38 -0700, Zach Brown wrote:
> On Jul 30, 2007, at 2:58 PM, Badari Pulavarty wrote:
> 
> > On Mon, 2007-07-30 at 14:45 -0700, Zach Brown wrote:
> >>> I am also taking a look at it right now.
> >>
> >> Are we having a race to write a little test app that reproduces the
> >> problem? :)
> >
> > Nope. Feel free to write the test case.
> 
> Well, I'm having a heck of a time getting this to fail.  It looks  
> possible, though.  Joe, were you guys able to narrow it down to a  
> reproducible test case?  Do you have any oops output messages from  
> the crashes?
> 
> It looks like it takes a very particular set of circumstances to  
> actually crash after relying on an uninitialized map_bh.  (see the  
> blkfactor, buffer_new(), and this_chunk_blocks tests in dio_zero_block 
> ()).


Looking at the crash

CPU:    0
EIP:    0060:[<c04a07fd>]    Not tainted VLI
EFLAGS: 00010293   (2.6.22 #2)
EIP is at bio_get_nr_vecs+0x0/0x30
eax: 23c07063   ebx: 00000003   ecx: ffffffff   edx: 00000000
esi: de5cef74   edi: f54a9600   ebp: 00000000   esp: de5ceca8
ds: 007b   es: 007b   fs: 00d8  gs: 0033  ss: 0068
Process fio (pid: 17820, ti=de5ce000 task=de6570e0 task.ti=de5ce000)
Stack: c04a1c9d ffffffff ffffffff 00000009 f54a9600 de5cef74 00000000
f54a9600
       c04a1f43 00000000 c04a2b46 c0460466 c2c5baa0 c0812500 c0462c0a
00000001
       00000001 df4b90d4 de5ceee4 00000011 00000001 00000009 00000009
00000000
Call Trace:
 [<c04a1c9d>] dio_new_bio+0x82/0xfe
 [<c04a1f43>] dio_send_cur_page+0x4a/0x92
 [<c04a2b46>] __blockdev_direct_IO+0xa09/0xc83
 [<c0460466>] __pagevec_free+0x14/0x1a
 [<c0462c0a>] release_pages+0x137/0x13f
 [<f8856f30>] journal_start+0xaf/0xdd [jbd]
 [<f8890fec>] ext3_direct_IO+0xfd/0x190 [ext3]
..

I am not sure, I really understand whats happening here. It looks
like dio->map_bh.b_dev is junk. But looking at the code, we shouldn't
be in dio_send_cur_page() unless if we have a valid dio->cur_page.
(which means, that we added a page to submit, which also means
previous getblock() succeded, which means that we should have a
valid map_bh). What am I missing here ?


        if (dio->cur_page) {
                ret2 = dio_send_cur_page(dio);
                if (ret == 0)
                        ret = ret2;
                page_cache_release(dio->cur_page);
                dio->cur_page = NULL;
        }

> 
> > I am just looking at the code
> > to see what needs to be done.
> 
> It looks like the unconditional dio_cleanup() and dio_zero_block()  
> calls outside the nseg loop are relying on state which might not have  
> been built up.  _zero_block() tests map_bh's flags without them being  
> set.  _cleanup could, in some crazy world, get confused if we managed  
> to get here with a 0 nr_segs because dio->head and ->tail wouldn't be  
> initialized.
> 
> So we could initialize some more fields at the start of  
> direct_io_worker for the benefit of these cleanup calls.  Or we could  
> conditionally call them based on some other indicator of progress.   
> Neither really thrills me.
> 
> And I don't have a test case to verify changes with.  Meh.
> 
> How do you feel about initializing the dio with kzalloc() and only  
> initializing the fields that we rely on being non-zero, and  
> commenting the hell out of it?

Yeah. kzalloc() may be right way to go.. But I would like to understand
what exactly is happening here.

Thanks,
Badari

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