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@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxx>
>> Cc: stable@xxxxxxxxxx
>> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
>> ---
>>
>> 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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