From 2cd1c8d4dc7ecca9e9431e2dabe41ae9c7d89e51 Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk Date: Tue, 15 Nov 2011 14:49:09 -0800 Subject: [PATCH 1/1] x86/paravirt: PTE updates in k(un)map_atomic need to be synchronous, regardless of lazy_mmu mode 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: [] 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:[] EFLAGS: 00210246 CPU: 3 EIP is at swap_count_continued+0x104/0x180 .. snip.. Call Trace: [] ? __swap_duplicate+0xc2/0x160 [] ? pte_mfn_to_pfn+0x87/0xe0 [] ? swap_duplicate+0x14/0x40 [] ? copy_pte_range+0x45b/0x500 [] ? copy_page_range+0x195/0x200 [] ? dup_mmap+0x1c6/0x2c0 [] ? dup_mm+0xa8/0x130 [] ? copy_process+0x98a/0xb30 [] ? do_fork+0x4f/0x280 [] ? getnstimeofday+0x43/0x100 [] ? sys_clone+0x30/0x40 [] ? ptregs_clone+0x15/0x48 [] ? 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 b8bcfe997e4615 ("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: Konrad Rzeszutek Wilk Cc: Peter Zijlstra Cc: Jeremy Fitzhardinge Cc: Signed-off-by: Andrew Morton Signed-off-by: Ingo Molnar --- arch/x86/mm/highmem_32.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/arch/x86/mm/highmem_32.c b/arch/x86/mm/highmem_32.c index b49962662101..f4f29b19fac5 100644 --- a/arch/x86/mm/highmem_32.c +++ b/arch/x86/mm/highmem_32.c @@ -45,6 +45,7 @@ void *kmap_atomic_prot(struct page *page, pgprot_t prot) vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx); BUG_ON(!pte_none(*(kmap_pte-idx))); set_pte(kmap_pte-idx, mk_pte(page, prot)); + arch_flush_lazy_mmu_mode(); return (void *)vaddr; } @@ -88,6 +89,7 @@ void __kunmap_atomic(void *kvaddr) */ kpte_clear_flush(kmap_pte-idx, vaddr); kmap_atomic_idx_pop(); + arch_flush_lazy_mmu_mode(); } #ifdef CONFIG_DEBUG_HIGHMEM else { -- 2.39.2