Re: [PATCH] add check do_direct_IO() return val

Badari Pulavarty
2007-07-31T00:13:32Z

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