suspect list_empty( {NULL, NULL} )

[Zach Brown]

A cute list_head debugging patch seems to have found strange list_entry
use in vmscan.c in stock 2.5.39.

page_mapping_inuse:

	if (!list_empty(&mapping->i_mmap) || !list_empty(&mapping->i_mmap_shared))

is being called for for a mapping in which:

 i_mmap = {next = 0x0, prev = 0x0}, i_mmap_shared = { next = 0x0, prev = 0x0}

this surely can't be intended.. list_empty will fail, but it isn't on
any lists.

I can reproduce this at will by cating a filesystem to /dev/null after
booting.  

too many details:

(gdb) bt
#0  0xc0138603 in shrink_list (page_list=0xc1b2fe94, nr_pages=-9, 
    gfp_mask=464, max_scan=0xc1b2ff0c, nr_mapped=0xc1b2ff44)
    at /var/tmp/zab/kernel/linux-2.5.39/include/linux/list.h:40
#1  0xc0138ff8 in shrink_cache (nr_pages=1, zone=0xc03b7000, gfp_mask=464, 
    max_scan=-31, nr_mapped=0xc1b2ff44) at vmscan.c:382
#2  0xc013a196 in shrink_zone (zone=0xc03b7000, max_scan=2, gfp_mask=464, 
    nr_pages=1, nr_mapped=0xc1b2ff44) at vmscan.c:555
#3  0xc013a20c in shrink_caches (classzone=0xc03b70dc, priority=12, 
    total_scanned=0xc1b2ff70, gfp_mask=464, nr_pages=0) at vmscan.c:593
#4  0xc013a296 in try_to_free_pages (classzone=0xc03b70dc, gfp_mask=464, 
    order=0) at vmscan.c:647
#5  0xc013a38f in kswapd_balance_pgdat (pgdat=0xc03b7000) at vmscan.c:695
#6  0xc013a406 in kswapd_balance () at vmscan.c:719
#7  0xc013a54c in kswapd (unused=0x0) at vmscan.c:810

 ( ftso inlines, damn right. )

(gdb) print *page
$21 = {flags = 9, count = {counter = 2}, list = {next = 0xc03b6f54, 
    prev = 0xc101b448}, mapping = 0xc03b6f40, index = 31, lru = {next = 0x0, 
    prev = 0x0}, pte = {chain = 0x0, direct = 0x0}, private = 0}

(gdb) print *mapping
$22 = {host = 0xc03b6e00, page_tree = {height = 1, gfp_mask = 32, 
    rnode = 0xc0775600}, page_lock = {gcc_is_buggy = 0}, clean_pages = {
    next = 0xc101b448, prev = 0xc100dd48}, dirty_pages = {next = 0xc03b6f5c, 
    prev = 0xc03b6f5c}, locked_pages = {next = 0xc1885e58, prev = 0xc1895790}, 
  io_pages = {next = 0xc03b6f6c, prev = 0xc03b6f6c}, nrpages = 32, 
  a_ops = 0xc03b6da0, i_mmap = {next = 0x0, prev = 0x0}, i_mmap_shared = {
    next = 0x0, prev = 0x0}, i_shared_lock = {gcc_is_buggy = 0}, 
  dirtied_when = 0, gfp_mask = 0, backing_dev_info = 0xc03b6f30, 
  private_lock = {gcc_is_buggy = 0}, private_list = {next = 0xc03b6fa0, 
    prev = 0xc03b6fa0}, assoc_mapping = 0x0}

(gdb) print page->mapping
$24 = (struct address_space *) 0xc03b6f40
(gdb) print &swapper_space
$25 = (struct address_space *) 0xc03b6f40

I am as about as ignorant about the VM as one can be.  I'm happy to
provide more info or act as a kgdb operator if this is actually
something that needs looking into..

- z