Re: bad page state under possibly oom situation

[Dipankar Sarma <dipankar@in.ibm.com>]

On Thu, Nov 03, 2005 at 07:38:09AM +0100, Manfred Spraul wrote:
> Hugh Dickins wrote:
> 
> >(I don't know that it makes any difference, but was this particular report
> >from 2.6.9-rc2 or from 2.6.14 or from something else?  In both 2.6.9 and
> >2.6.14, flags 0x90 mean PG_slab|PG_dirty.)
> >
> A very odd combination:
> - free_pages_check() ensures that neither PG_slab nor PG_dirty are set
> - prep_new_page() complains that both PG_slab and PG_dirty are set
> - AFAICS slab doesn't set PG_dirty, and noone except slab set PG_slab.
> 
> I don't understand how two wrong bits can end up in page->flags.
> Dipankar, could you modify bad_page() and hexdump +-128 bytes? Perhaps 
> someone overwrites random memory. Or change the value of PG_slab to 20 
> and check if page->flags remains 0x90.

Here is a dump of the page struct when this happens (two different
instances) - 

/* Dump of struct page */
page = ffff810008005550
4000005500009090        ffffffffffffffff        0       0
0       100100  200200  4000000000000000
ffffffffffffffff        0       0       0
ffff8100080055e8        ffffffff805ba370        4000000000000000        ffffffffffffffff

Bad page state at prep_new_page (in process 'rename14', page ffff810008005550)
flags:0x4000005500009090 mapping:0000000000000000 mapcount:0 count:0
Backtrace:

Call Trace:<ffffffff80150388>{bad_page+115} <ffffffff80150bf0>{buffered_rmqueue+501}
       <ffffffff8017e2da>{alloc_inode+18} <ffffffff80150de7>{__alloc_pages+251}
       <ffffffff801535f3>{cache_alloc_refill+581} <ffffffff8015388f>{kmem_cache_alloc+44}
       <ffffffff80169440>{get_empty_filp+71} <ffffffff801670af>{filp_open+49}
       <ffffffff80166e28>{get_unused_fd+98} <ffffffff8016713d>{do_sys_open+59}
       <ffffffff8010e636>{system_call+126}
Trying to fix it up, but a reboot is needed
page = ffff81000800aaa0
4000000000000000        ffffffff00900055        0       0
0       100100  200200  4000000000000000
ffffffffffffffff        0       0       0
ffff81000800ab38        ffffffff805ba370        4000000000000000        ffffffffffffffff
Bad page state at prep_new_page (in process 'rename14', page ffff81000800aaa0)
flags:0x4000000000000000 mapping:0000000000000000 mapcount:0 count:9437270
Backtrace:

Call Trace:<ffffffff80150388>{bad_page+115} <ffffffff80150bf0>{buffered_rmqueue+501}
       <ffffffff80150de7>{__alloc_pages+251} <ffffffff801535f3>{cache_alloc_refill+581}
       <ffffffff8015388f>{kmem_cache_alloc+44} <ffffffff8017cd95>{d_alloc+33}
       <ffffffff801750be>{__lookup_hash+206} <ffffffff8017677b>{open_namei+276}
       <ffffffff801670ce>{filp_open+80} <ffffffff80166e28>{get_unused_fd+98}
       <ffffffff8016713d>{do_sys_open+59} <ffffffff8010e636>{system_call+1

I had set PG_slab to 20, so it is not necessarily a corrupted slab
page.

Thanks
Dipankar