x86/paravirt: PTE updates in k(un)map_atomic need to be synchronous, regardless of...

x86/paravirt: PTE updates in k(un)map_atomic need to be synchronous, regardless of lazy_mmu mode.

This patch fixes an outstanding issue that has been reported since 2.6.37.
Under a heavy loaded machine processing "fork()" calls could keepover with:

BUG: unable to handle kernel paging request at f573fc8c
IP: [<c01abc54>] swap_count_continued+0x104/0x180
*pdpt = 000000002a3b9027 *pde = 0000000001bed067 *pte = 0000000000000000
Oops: 0000 [#1] SMP
Modules linked in:
Pid: 1638, comm: apache2 Not tainted 3.0.4-linode37 #1
EIP: 0061:[<c01abc54>] EFLAGS: 00210246 CPU: 3
EIP is at swap_count_continued+0x104/0x180
.. snip..
Call Trace:
 [<c01ac222>] ? __swap_duplicate+0xc2/0x160
 [<c01040f7>] ? pte_mfn_to_pfn+0x87/0xe0
 [<c01ac2e4>] ? swap_duplicate+0x14/0x40
 [<c01a0a6b>] ? copy_pte_range+0x45b/0x500
 [<c01a0ca5>] ? copy_page_range+0x195/0x200
 [<c01328c6>] ? dup_mmap+0x1c6/0x2c0
 [<c0132cf8>] ? dup_mm+0xa8/0x130
 [<c013376a>] ? copy_process+0x98a/0xb30
 [<c013395f>] ? do_fork+0x4f/0x280
 [<c01573b3>] ? getnstimeofday+0x43/0x100
 [<c010f770>] ? sys_clone+0x30/0x40
 [<c06c048d>] ? ptregs_clone+0x15/0x48
 [<c06bfb71>] ? syscall_call+0x7/0xb

The problem is that in copy_page_range we turn lazy mode on, and then
in swap_entry_free we call swap_count_continued which ends up in:

         map = kmap_atomic(page, KM_USER0) + offset;

and then later we touch *map.

Since we are running in batched mode (lazy) we don't actually set up the
PTE mappings and the kmap_atomic is not done synchronously and ends up
trying to dereference a page that has not been set.

Looking at kmap_atomic_prot_pfn, it uses 'arch_flush_lazy_mmu_mode' and
doing the same in kmap_atomic_prot and __kunmap_atomic makes the problem
go away.

Interestingly, git commit b8bcfe997e46150fedcc3f5b26b846400122fdd9
removed part of this to fix an interrupt issue - but it went to far
and did not consider this scenario.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: x86@kernel.org
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
CC: stable@kernel.org
[v1: Redid the commit description per Jeremy's apt suggestion]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>