• Konrad Rzeszutek Wilk's avatar
    x86/paravirt: PTE updates in k(un)map_atomic need to be synchronous, regardless of lazy_mmu mode · 2cd1c8d4
    Konrad Rzeszutek Wilk authored
    Fix an outstanding issue that has been reported since 2.6.37.
    Under a heavy loaded machine processing "fork()" calls could
    crash 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, commit b8bcfe99 ("x86/paravirt: remove lazy
    mode in interrupts") removed part of this to fix an interrupt
    issue - but it went to far and did not consider this scenario.
    Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
    Cc: <stable@kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    2cd1c8d4
highmem_32.c 3.33 KB