Re: Oopses with both recent 2.4.x kernels and 2.6.x kernels

[Steven Dake]

On Tue, 2004-06-15 at 06:16, Marcelo Tosatti wrote:
> On Mon, Jun 14, 2004 at 10:07:05AM -0700, Steven Dake wrote:
> > Marcelo and Stian,
> > 
> > I have also seen this oops relating to low memory situations.  I think
> > ext3 allocates some data, has a null return, sets something to null, and
> > then later it is dereferenced in kwapd.
> > 
> > Anyone have a patch for this problem?

> Steven, 
> 
> For what I remember Stian oopses were happening in random places in the VM freeing 
> routines. That makes me belive what he was seeing was some kind of hardware issue, 
> because otherwise the oopses would be happening in the same place (in case it was 
> a software bug). The codepaths which he saw trying to access invalid addresses are 
> executed flawlessly by all 2.4.x mainline users. He was also seeing oopses with v2.6.
> 
> Assuming his HW is not faulty, I can think of some driver corrupting his memory. 
> 
> Do you have any traces of the oopses you are seeing?  
> 
> Stian, you told us switched servers now, I assume the problem is gone? 
> Are you still running v2.4 on that server?
> 

Marcelo,

Stian responded saying he upgraded to 2.6 and also removed the memory
intensive script and his problems went away.  I suspect removing the
memory intesive script did the trick.  2.6 could also be fixed, who
knows?

After reading lkml, there are about 4-5 Oops in the same function at the
same location.  The people report they were heavily using memory (low
memory situation) in some of the bug reports.

I have tracked down the problem to a null dereference during a buffer
cache rebalance (which occurs during low memory situations).  Here is
the info I have.  Unfortunately I don't know much about how vm handles
low memory situations with the vfs, so if you have any ideas, it would
be helpful. :)

nable to handle kernel NULL pointer dereference at virtual address
00000028
<4> printing eip:
<4>c018aa67
<1>*pde = 00000000
<4>Oops: 0000
<4>CPU:	 2
<4>EIP:	 0010:[<c018aa67>]    Not tainted
<4>EFLAGS: 00010203
<4>eax: 00000000	 ebx: 00000000	 ecx: c0380490	 edx: c217a540
<4>esi: f2757a80	 edi: 00000001	 ebp: 00000000	 esp: f7bd1ee8
<4>ds: 0018   es: 0018   ss: 0018
<4>Process kswapd (pid: 11, stackpage=f7bd1000)
<4>Stack: 00000000 00000000 c217a600 00000000 00000202 00000000 c217a540 000001d0
<4>	c217a540 000102ec c018144f f7abe400 c217a540 000001d0 c0155006 c217a540
<4>	000001d0 f7bd0000 00000000 c0147f0a c217a540 000001d0 f7bd0000 f7bd0000
<4>Call Trace: [<c018144f>] [<c0155006>] [<c0147f0a>]
[<c014832b>] [<c0148398>]
<4>   [<c0148431>] [<c014847f>] [<c0148593>] [<c0105000>]
[<c010578a>] [<c01484f8>]
<4>
<4>Code: 8b 5b 28 f6 40 19 02 75 47 39 f3 75 f1 c6 05 80 24 38 c0 01


>>EIP; c018aa67 <journal_try_to_free_buffers+45/f4>   <=====
Trace; c018144f <ext3_releasepage+2d/32>
Trace; c0155006 <try_to_release_page+4e/78>
Trace; c0147f0a <shrink_cache+270/4f6>
Trace; c014832b <shrink_caches+61/98>
Trace; c0148398 <try_to_free_pages+36/54>
Trace; c0148431 <kswapd_balance_pgdat+51/8a>
Trace; c014847f <kswapd_balance+15/2c>
Trace; c0148593 <kswapd+9b/b6>
Trace; c0105000 <_stext+0/0>
Trace; c010578a <kernel_thread+2e/40>
Trace; c01484f8 <kswapd+0/b6>

The problem is that in this function:

int journal_try_to_free_buffers(journal_t *journal,
		struct page *page, int gfp_mask)
{
    struct buffer_head *bh;
    struct buffer_head *tmp;
    int locked_or_dirty = 0;
    int call_ttfb = 1;

    J_ASSERT(PageLocked(page));

    bh = page->buffers;
    tmp = bh;
    spin_lock(&journal_datalist_lock);
    do {
	struct buffer_head *p = tmp;

	tmp = tmp->b_this_page;
    ...

tmp (page->buffers) above is null.  b_this_page is at offset 0x28 (the accessed address in the oops).  This means that
page->buffers is set to null by some other routine which results in the oops.

I read the page allocate code
(ext3_read_page->block_read_full_page->create_emty_buffers->create_buffers), and it appears that it is not possible to allocate a page->buffers value of zero in the allocate function.  I am having difficulty reproducing and cannot debug further, however.  Can page->buffers be set to zero somewhere else?  Perhaps kswapd and some other thread are racing on the free?

Thansk
-steve
> Thanks!

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