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SubjectOne (possible) x86 get_user_pages bug
Actually this bug is met with a SLES11 SP1 dom0 kernel 
(2.6.32.12-0.7-xen), and we still can't reproduce it with a native
2.6.32 kernel. But as we suspect the native kernel might have the same
issue, we send it to LKML for consultant.

At first the error message looks like this:
----------------------------------------------------------------
[201674.150162] BUG: Bad page state in process java pfn:d13b8
[201674.151345] page:ffff8800075c7040 flags:4000000000200000 count:0
mapcount:0 mapping:(null) index:7f093bdfd
[201674.152474] Pid: 14793, comm: java Tainted: G N
2.6.32.12-0.7-xen #2
[201674.153585] Call Trace:
[201674.154643] [<ffffffff80009a75>] dump_trace+0x65/0x180
[201674.155686] [<ffffffff80369f26>] dump_stack+0x69/0x73
[201674.156744] [<ffffffff8009c0df>] bad_page+0xdf/0x160
[201674.157773] [<ffffffff800614f1>] get_futex_key+0x71/0x1a0
[201674.158820] [<ffffffff80061f72>] futex_wake+0x52/0x130
[201674.159852] [<ffffffff8006414f>] do_futex+0x11f/0xc40
[201674.160875] [<ffffffff80064cf2>] sys_futex+0x82/0x160
[201674.161907] [<ffffffff8003aa26>] mm_release+0xb6/0x110
[201674.162960] [<ffffffff8003f65e>] exit_mm+0x1e/0x150
[201674.163991] [<ffffffff80040567>] do_exit+0x127/0x7e0
[201674.165028] [<ffffffff80040c32>] sys_exit+0x12/0x20
[201674.166070] [<ffffffff80007388>] system_call_fastpath+0x16/0x1b
[201674.167130] [<00007f098db046b0>] 0x7f098db046b0
----------------------------------------------------------------
After CONFIG_DEBUG_VM option turned on (kind of), the faulting spot is
captured -- get_page() in gup_pte_range() is used upon a free page and
it triggers a BUG_ON.

We created a scenario to reproduce the bug:
----------------------------------------------------------------
// proc1/proc1.2 are 2 threads sharing one page table.
// proc1 is the parent of proc2.
proc1 proc2 proc1.2
... ... // in gup_pte_range()
... ... pte = gup_get_pte()
... ... page1 = pte_page(pte) // (1)
do_wp_page(page1) ... ...
... exit_map() ...
... ... get_page(page1) // (2)
-----------------------------------------------------------------
do_wp_page() and exit_map() cause page1 to be released into free list
before get_page() in proc1.2 is called. The longer the delay between
(1)&(2), the easier the BUG_ON shows.

An experimental patch is made to prevent the PTE being modified in the
middle of gup_pte_range(). The BUG_ON disappears afterward.

However, from the comments embedded in gup.c, it seems deliberate to
avoid the lock in the fast path. The question is: if so, how to avoid
the above scenario?

Thanks,
xiaowei

--------------------------------------------------------------------
--- /usr/src/linux-2.6.32.12-0.7/arch/x86/mm/gup.c.org 2011-01-27
20:11:45.000000000 +0800
+++ /usr/src/linux-2.6.32.12-0.7/arch/x86/mm/gup.c 2011-01-27
20:11:22.000000000 +0800
@@ -72,17 +72,18 @@static noinline int gup_pte_range(pmd_t pmd, unsigned
long addr,
unsigned long end, int write, struct
page **pages, int *nr)
{
unsigned long mask;
pte_t *ptep;
+ spinlock_t *ptl;

mask = _PAGE_PRESENT|_PAGE_USER;
if (write)
mask |= _PAGE_RW;
- ptep = pte_offset_map(&pmd, addr);
+ ptep = pte_offset_map_lock(current->mm, &pmd, addr, &ptl);
do {
pte_t pte = gup_get_pte(ptep);
struct page *page;

if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
- pte_unmap(ptep);
+ pte_unmap_unlock(ptep, ptl);
return 0;
}
VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
@@ -90,8 +91,9 @@
get_page(page);
pages[*nr] = page;
(*nr)++;

} while (ptep++, addr += PAGE_SIZE, addr != end);
- pte_unmap(ptep - 1);
+ pte_unmap_unlock(ptep - 1, ptl);

return 1;
}



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