Re: [PATCH] proc/pagemap: walk page tables under pte lock

From: Naoya Horiguchi
Date: Mon Jan 26 2015 - 20:54:06 EST

On Mon, Jan 26, 2015 at 05:52:14PM +0300, Konstantin Khlebnikov wrote:
> Lockless access to pte in pagemap_pte_range() might race with page migration
> and trigger BUG_ON(!PageLocked()) in migration_entry_to_page():
>
> CPU A (pagemap) CPU B (migration)
> lock_page()
> try_to_unmap(page, TTU_MIGRATION...)
> make_migration_entry()
> set_pte_at()
> <read *pte>
> pte_to_pagemap_entry()
> remove_migration_ptes()
> unlock_page()
> if(is_migration_entry())
> migration_entry_to_page()
> BUG_ON(!PageLocked(page))
>
> Also lockless read might be non-atomic if pte is larger than wordsize.
> Other pte walkers (smaps, numa_maps, clear_refs) already lock ptes.
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Reported-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
> Fixes: 052fb0d635df ("proc: report file/anon bit in /proc/pid/pagemap")
> Cc: Stable <stable@xxxxxxxxxxxxxxx> (v3.5+)

Acked-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>

> ---
>
> ------------[ cut here ]------------
> kernel BUG at ../include/linux/swapops.h:131!
> invalid opcode: 0000 [#1] PREEMPT SMP
> Modules linked in:
> CPU: 0 PID: 702 Comm: a.out Not tainted 3.19.0-rc1+ #16
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org
> 04/01/2014
> task: ffff880001dc0dc0 ti: ffff88003dbd0000 task.ti: ffff88003dbd0000
> RIP: pagemap_pte_range (include/linux/swapops.h:131 fs/proc/task_mmu.c:1136)
> RSP: 0018:ffff88003dbd3d08 EFLAGS: 00010246
> RAX: ffffea0000492b00 RBX: ffff88003dbd3e60 RCX: ffff880013bbe878
> RDX: 0000000000000000 RSI: 000000000000001f RDI: 3e00000000000000
> RBP: ffff88003dbd3d78 R08: 000000000024959f R09: ffff880001f5d000
> R10: 0000000000000000 R11: 007fffffffffffff R12: 00007f6662000000
> R13: ffff880001f5d000 R14: 00007f6661e8f000 R15: 0600000000000000
> FS: 00007f666381d700(0000) GS:ffff88003fc00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f666333a520 CR3: 0000000013b7f000 CR4: 00000000000407b0
> Stack:
> 0000000000000000 0000000062000000 ffff88003dbd3e78 ffff880013bbe878
> 007fffffffffffff 000000000003f65d ffffffff810d3710 0000000000000010
> ffff88003dbd3d68 ffff88003dbd3e78 00007f6662000000 00007f6661e62000
> Call Trace:
> ? find_vma (mm/mmap.c:2046)
> walk_page_range (mm/pagewalk.c:51 mm/pagewalk.c:92 mm/pagewalk.c:241)
> pagemap_read (fs/proc/task_mmu.c:1297)
> ? pid_maps_open (fs/proc/task_mmu.c:1068)
> ? m_stop (kernel/module.c:621)
> __vfs_read (fs/read_write.c:430)
> vfs_read (fs/read_write.c:455)
> SyS_pread64 (fs/read_write.c:619 fs/read_write.c:606)
> system_call_fastpath (arch/x86/kernel/entry_64.S:423)
> Code: a0 4c 89 f6 48 8b 78 30 e8 a1 10 f9 ff 48 8b 4d b8 49 89 c4 e9
> 6d fc ff ff 0f 1f 44 00 00 49 39 ce 73 17 48 89 cf e9 7b fc ff ff <0f>
> 0b 4c 89 c7 e8 a9 bc f9 ff e9 ea fb ff ff 44 8b 55 9c 44 89
> All code
> ========
> 0: a0 4c 89 f6 48 8b 78 movabs 0xe830788b48f6894c,%al
> 7: 30 e8
> 9: a1 10 f9 ff 48 8b 4d movabs 0x49b84d8b48fff910,%eax
> 10: b8 49
> 12: 89 c4 mov %eax,%esp
> 14: e9 6d fc ff ff jmpq 0xfffffffffffffc86
> 19: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> 1e: 49 39 ce cmp %rcx,%r14
> 21: 73 17 jae 0x3a
> 23: 48 89 cf mov %rcx,%rdi
> 26: e9 7b fc ff ff jmpq 0xfffffffffffffca6
> 2b:* 0f 0b ud2 <-- trapping instruction
> 2d: 4c 89 c7 mov %r8,%rdi
> 30: e8 a9 bc f9 ff callq 0xfffffffffff9bcde
> 35: e9 ea fb ff ff jmpq 0xfffffffffffffc24
> 3a: 44 8b 55 9c mov -0x64(%rbp),%r10d
> 3e: 44 rex.R
> 3f: 89 .byte 0x89
>
> Code starting with the faulting instruction
> ===========================================
> 0: 0f 0b ud2
> 2: 4c 89 c7 mov %r8,%rdi
> 5: e8 a9 bc f9 ff callq 0xfffffffffff9bcb3
> a: e9 ea fb ff ff jmpq 0xfffffffffffffbf9
> f: 44 8b 55 9c mov -0x64(%rbp),%r10d
> 13: 44 rex.R
> 14: 89 .byte 0x89
> RIP pagemap_pte_range (include/linux/swapops.h:131 fs/proc/task_mmu.c:1136)
> RSP <ffff88003dbd3d08>
> ------------[ cut here ]------------
> ---
> fs/proc/task_mmu.c | 14 +++++++++-----
> 1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 2464367..ff65557 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1070,7 +1070,7 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> struct vm_area_struct *vma;
> struct pagemapread *pm = walk->private;
> spinlock_t *ptl;
> - pte_t *pte;
> + pte_t *pte, *orig_pte;
> int err = 0;
>
> /* find the first VMA at or above 'addr' */
> @@ -1131,15 +1131,19 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> BUG_ON(is_vm_hugetlb_page(vma));
>
> /* Addresses in the VMA. */
> - for (; addr < min(end, vma->vm_end); addr += PAGE_SIZE) {
> + orig_pte = pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> + for (; addr < min(end, vma->vm_end); pte++, addr += PAGE_SIZE) {
> pagemap_entry_t pme;
> - pte = pte_offset_map(pmd, addr);
> +
> pte_to_pagemap_entry(&pme, pm, vma, addr, *pte);
> - pte_unmap(pte);
> err = add_to_pagemap(addr, &pme, pm);
> if (err)
> - return err;
> + break;
> }
> + pte_unmap_unlock(orig_pte, ptl);
> +
> + if (err)
> + return err;
>
> if (addr == end)
> break;
> --
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