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