Re: [PATCH v1] mm/hwpoison: set PageHWPoison after taking page lock in memory_failure_hugetlb()

[Yang Shi <shy828301@gmail.com>]

On Wed, Mar 9, 2022 at 1:15 AM Naoya Horiguchi
<naoya.horiguchi@linux.dev> wrote:
>
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>
> There is a race condition between memory_failure_hugetlb() and hugetlb
> free/demotion, which causes setting PageHWPoison flag on the wrong page
> (which was a hugetlb when memory_failrue() was called, but was removed
> or demoted when memory_failure_hugetlb() is called).  This results in
> killing wrong processes.  So set PageHWPoison flag with holding page lock,
>
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> ---
>  mm/memory-failure.c | 27 ++++++++++++---------------
>  1 file changed, 12 insertions(+), 15 deletions(-)
>
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index ac6492e36978..fe25eee8f9d6 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1494,24 +1494,11 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
>         int res;
>         unsigned long page_flags;
>
> -       if (TestSetPageHWPoison(head)) {
> -               pr_err("Memory failure: %#lx: already hardware poisoned\n",
> -                      pfn);
> -               res = -EHWPOISON;
> -               if (flags & MF_ACTION_REQUIRED)
> -                       res = kill_accessing_process(current, page_to_pfn(head), flags);
> -               return res;
> -       }
> -
> -       num_poisoned_pages_inc();
> -
>         if (!(flags & MF_COUNT_INCREASED)) {
>                 res = get_hwpoison_page(p, flags);

I'm not an expert of hugetlb, I may be wrong. I'm wondering how this
could solve the race? Is the below race still possible?

__get_hwpoison_page()
  head = compound_head(page)

hugetlb demotion (1G --> 2M)
  get_hwpoison_huge_page(head, &hugetlb);


Then the head may point to a 2M page, but the hwpoisoned subpage is
not in that 2M range?


>                 if (!res) {
>                         lock_page(head);
>                         if (hwpoison_filter(p)) {
> -                               if (TestClearPageHWPoison(head))
> -                                       num_poisoned_pages_dec();
>                                 unlock_page(head);
>                                 return -EOPNOTSUPP;
>                         }
> @@ -1544,13 +1531,16 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
>         page_flags = head->flags;
>
>         if (hwpoison_filter(p)) {
> -               if (TestClearPageHWPoison(head))
> -                       num_poisoned_pages_dec();
>                 put_page(p);
>                 res = -EOPNOTSUPP;
>                 goto out;
>         }
>
> +       if (TestSetPageHWPoison(head))

And I don't think "head" is still the head you expected if the race
happened. I think we need to re-retrieve the head once the page
refcount is bumped and locked.

> +               goto already_hwpoisoned;
> +
> +       num_poisoned_pages_inc();
> +
>         /*
>          * TODO: hwpoison for pud-sized hugetlb doesn't work right now, so
>          * simply disable it. In order to make it work properly, we need
> @@ -1576,6 +1566,13 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
>  out:
>         unlock_page(head);
>         return res;
> +already_hwpoisoned:
> +       unlock_page(head);
> +       pr_err("Memory failure: %#lx: already hardware poisoned\n", pfn);
> +       res = -EHWPOISON;
> +       if (flags & MF_ACTION_REQUIRED)
> +               res = kill_accessing_process(current, page_to_pfn(head), flags);
> +       return res;
>  }
>
>  static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
> --
> 2.25.1
>