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SubjectRe: [PATCH 02/10] mm,madvise: call soft_offline_page() without MF_COUNT_INCREASED
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On 10.09.19 12:30, Oscar Salvador wrote:
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>
> Currently madvise_inject_error() pins the target via get_user_pages_fast.
> The call to get_user_pages_fast is only to get the respective page
> of a given address, but it is the job of the memory-poisoning handler
> to deal with races, so drop the refcount grabbed by get_user_pages_fast.
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
> mm/madvise.c | 25 +++++++++++--------------
> 1 file changed, 11 insertions(+), 14 deletions(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 6e023414f5c1..fbe6d402232c 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -883,6 +883,16 @@ static int madvise_inject_error(int behavior,
> ret = get_user_pages_fast(start, 1, 0, &page);
> if (ret != 1)
> return ret;
> + /*
> + * The get_user_pages_fast() is just to get the pfn of the
> + * given address, and the refcount has nothing to do with
> + * what we try to test, so it should be released immediately.
> + * This is racy but it's intended because the real hardware
> + * errors could happen at any moment and memory error handlers
> + * must properly handle the race.
> + */
> + put_page(page);
> +

I wonder if it would be clearer to do that after the page has been fully
used - e.g. after getting the pfn and the order (and then e.g.,
symbolically setting the page pointer to 0).

I guess the important part of this patch is to not have an elevated
refcount while calling soft_offline_page().

> pfn = page_to_pfn(page);
>
> /*
> @@ -892,16 +902,11 @@ static int madvise_inject_error(int behavior,
> */
> order = compound_order(compound_head(page));
>
> - if (PageHWPoison(page)) {
> - put_page(page);
> - continue;
> - }

This change is not reflected in the changelog. I would have expected
that only the put_page() would go. If this should go completely, I
suggest a separate patch.

> -
> if (behavior == MADV_SOFT_OFFLINE) {
> pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n",
> pfn, start);
>
> - ret = soft_offline_page(page, MF_COUNT_INCREASED);
> + ret = soft_offline_page(page, 0);
> if (ret)
> return ret;
> continue;
> @@ -909,14 +914,6 @@ static int madvise_inject_error(int behavior,
>
> pr_info("Injecting memory failure for pfn %#lx at process virtual address %#lx\n",
> pfn, start);
> -
> - /*
> - * Drop the page reference taken by get_user_pages_fast(). In
> - * the absence of MF_COUNT_INCREASED the memory_failure()
> - * routine is responsible for pinning the page to prevent it
> - * from being released back to the page allocator.
> - */
> - put_page(page);
> ret = memory_failure(pfn, 0);
> if (ret)
> return ret;
>


--

Thanks,

David / dhildenb

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