Re: kernel 3.0: BUG: soft lockup: find_get_pages+0x51/0x110

[Hugh Dickins <hughd@google.com>]

I've not read through and digested Andrea's reply yet, but I'd say
this is not something we need to rush into 3.1 at the last moment,
before it's been fully considered: the bug here is hard to hit,
ancient, made more likely in 2.6.35 by compaction and in 2.6.38 by
THP's reliance on compaction, but not a regression in 3.1 at all - let
it wait until stable.

Hugh

On Sun, Oct 16, 2011 at 3:37 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> What's the status of this thing? Is it stable/3.1 material? Do we have
> ack/nak's for it? Anybody?
>
>                               Linus
>
> On Thu, Oct 13, 2011 at 4:16 PM, Hugh Dickins <hughd@google.com> wrote:
>>
>> [PATCH] mm: add anon_vma locking to mremap move
>>
>> I don't usually pay much attention to the stale "? " addresses in
>> stack backtraces, but this lucky report from Pawel Sikora hints that
>> mremap's move_ptes() has inadequate locking against page migration.
>>
>>  3.0 BUG_ON(!PageLocked(p)) in migration_entry_to_page():
>>  kernel BUG at include/linux/swapops.h:105!
>>  RIP: 0010:[<ffffffff81127b76>]  [<ffffffff81127b76>]
>>                       migration_entry_wait+0x156/0x160
>>  [<ffffffff811016a1>] handle_pte_fault+0xae1/0xaf0
>>  [<ffffffff810feee2>] ? __pte_alloc+0x42/0x120
>>  [<ffffffff8112c26b>] ? do_huge_pmd_anonymous_page+0xab/0x310
>>  [<ffffffff81102a31>] handle_mm_fault+0x181/0x310
>>  [<ffffffff81106097>] ? vma_adjust+0x537/0x570
>>  [<ffffffff81424bed>] do_page_fault+0x11d/0x4e0
>>  [<ffffffff81109a05>] ? do_mremap+0x2d5/0x570
>>  [<ffffffff81421d5f>] page_fault+0x1f/0x30
>>
>> mremap's down_write of mmap_sem, together with i_mmap_mutex/lock,
>> and pagetable locks, were good enough before page migration (with its
>> requirement that every migration entry be found) came in; and enough
>> while migration always held mmap_sem.  But not enough nowadays, when
>> there's memory hotremove and compaction: anon_vma lock is also needed,
>> to make sure a migration entry is not dodging around behind our back.
>>
>> It appears that Mel's a8bef8ff6ea1 "mm: migration: avoid race between
>> shift_arg_pages() and rmap_walk() during migration by not migrating
>> temporary stacks" was actually a workaround for this in the special
>> common case of exec's use of move_pagetables(); and we should probably
>> now remove that VM_STACK_INCOMPLETE_SETUP stuff as a separate cleanup.
>>
>> Reported-by: Pawel Sikora <pluto@agmk.net>
>> Cc: stable@kernel.org
>> Signed-off-by: Hugh Dickins <hughd@google.com>
>> ---
>>
>>  mm/mremap.c |    5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> --- 3.1-rc9/mm/mremap.c 2011-07-21 19:17:23.000000000 -0700
>> +++ linux/mm/mremap.c   2011-10-13 14:36:25.097780974 -0700
>> @@ -77,6 +77,7 @@ static void move_ptes(struct vm_area_str
>>                unsigned long new_addr)
>>  {
>>        struct address_space *mapping = NULL;
>> +       struct anon_vma *anon_vma = vma->anon_vma;
>>        struct mm_struct *mm = vma->vm_mm;
>>        pte_t *old_pte, *new_pte, pte;
>>        spinlock_t *old_ptl, *new_ptl;
>> @@ -95,6 +96,8 @@ static void move_ptes(struct vm_area_str
>>                mapping = vma->vm_file->f_mapping;
>>                mutex_lock(&mapping->i_mmap_mutex);
>>        }
>> +       if (anon_vma)
>> +               anon_vma_lock(anon_vma);
>>
>>        /*
>>         * We don't have to worry about the ordering of src and dst
>> @@ -121,6 +124,8 @@ static void move_ptes(struct vm_area_str
>>                spin_unlock(new_ptl);
>>        pte_unmap(new_pte - 1);
>>        pte_unmap_unlock(old_pte - 1, old_ptl);
>> +       if (anon_vma)
>> +               anon_vma_unlock(anon_vma);
>>        if (mapping)
>>                mutex_unlock(&mapping->i_mmap_mutex);
>>        mmu_notifier_invalidate_range_end(vma->vm_mm, old_start, old_end);
>

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