Re: [PATCH 3/7] mm,madvise: call soft_offline_page() without MF_COUNT_INCREASED

[Vlastimil Babka <vbabka@suse.cz>]

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);
>   		}
>   
>