Re: [PATCH] fix unmap_vmas() with NULL vma

[Akinobu Mita]

> Why bail out this late?  We can save the other stuff in exit_mmap() as
> well if we have no mmaps.
>
> Granted, the path is dead cold so the extra call overhead doesn't
> matter but I think the check is logically better placed in
> exit_mmap().

Looks good, this patch should go in.

>        Hannes
>
> ---
> Subject: mm: check for no mmaps in exit_mmap()
>
> When dup_mmap() ooms we can end up with mm->mmap == NULL.  The error
> path does mmput() and unmap_vmas() gets a NULL vma which it
> dereferences.
>
> In exit_mmap() there is nothing to do at all for this case, we can
> cancel the callpath right there.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d4855a6..b9d1636 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2091,6 +2091,9 @@ void exit_mmap(struct mm_struct *mm)
>        arch_exit_mmap(mm);
>        mmu_notifier_release(mm);
>
> +       if (!mm->mmap)
> +               return;
> +
>        if (mm->locked_vm) {
>                vma = mm->mmap;
>                while (vma) {
>
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