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SubjectRe: [PATCH 3/7] mm,madvise: call soft_offline_page() without MF_COUNT_INCREASED
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On 11/19/20 11:57 AM, Oscar Salvador wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>
> The call to get_user_pages_fast is only to get the pointer to a struct
> page of a given address, pinning it is memory-poisoning handler's job,
> so drop the refcount grabbed by get_user_pages_fast().
>
> Note that the target page is still pinned after this put_page() because
> the current process should have refcount from mapping.

Well, but can't it go away due to reclaim, migration or whatever?

> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
> mm/madvise.c | 19 +++++++++++--------
> 1 file changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index c6b5524add58..7a0f64b93635 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -900,20 +900,23 @@ static int madvise_inject_error(int behavior,
> */
> size = page_size(compound_head(page));
>
> + /*
> + * 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.

Sure they have to. We might just be unexpectedly messing with other process'
memory. Or does anything else prevent that?

> + */
> + put_page(page);
> +
> if (behavior == MADV_SOFT_OFFLINE) {
> pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n",
> pfn, start);
> - ret = soft_offline_page(pfn, MF_COUNT_INCREASED);
> + ret = soft_offline_page(pfn, 0);
> } else {
> 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);
> }
>
>

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