Re: [PATCH v1] mm,hwpoison: set PG_hwpoison for busy hugetlb pages

[Mike Kravetz]

On 5/11/22 08:19, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> 
> If memory_failure() fails to grab page refcount on a hugetlb page
> because it's busy, it returns without setting PG_hwpoison on it.
> This not only loses a chance of error containment, but breaks the rule
> that action_result() should be called only when memory_failure() do
> any of handling work (even if that's just setting PG_hwpoison).
> This inconsistency could harm code maintainability.
> 
> So set PG_hwpoison and call hugetlb_set_page_hwpoison() for such a case.
> 
> Fixes: 405ce051236c ("mm/hwpoison: fix race between hugetlb free/demotion and memory_failure_hugetlb()")
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> ---
>  include/linux/mm.h  | 1 +
>  mm/memory-failure.c | 8 ++++----
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d446e834a3e5..04de0c3e4f9f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3187,6 +3187,7 @@ enum mf_flags {
>  	MF_MUST_KILL = 1 << 2,
>  	MF_SOFT_OFFLINE = 1 << 3,
>  	MF_UNPOISON = 1 << 4,
> +	MF_NO_RETRY = 1 << 5,
>  };
>  extern int memory_failure(unsigned long pfn, int flags);
>  extern void memory_failure_queue(unsigned long pfn, int flags);
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 6a28d020a4da..e3269b991016 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1526,7 +1526,8 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
>  			count_increased = true;
>  	} else {
>  		ret = -EBUSY;
> -		goto out;
> +		if (!(flags & MF_NO_RETRY))
> +			goto out;
>  	}

Hi Naoya,

We are in the else block because !HPageFreed() and !HPageMigratable().
IIUC, this likely means the page is isolated.  One common reason for isolation
is migration.  So, the page could be isolated and on a list for migration.

I took a quick look at the hugetlb migration code and did not see any checks
for PageHWPoison after a hugetlb page is isolated.  I could have missed
something?  If there are no checks, we will read the PageHWPoison page
in kernel mode while copying to the migration target.

Is this an issue?  Is is something we need to be concerned with?  Memory
errors can happen at any time, and gracefully handling them is best effort.
-- 
Mike Kravetz

>  
>  	if (TestSetPageHWPoison(head)) {
> @@ -1556,7 +1557,6 @@ static int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb
>  	struct page *p = pfn_to_page(pfn);
>  	struct page *head;
>  	unsigned long page_flags;
> -	bool retry = true;
>  
>  	*hugetlb = 1;
>  retry:
> @@ -1572,8 +1572,8 @@ static int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb
>  		}
>  		return res;
>  	} else if (res == -EBUSY) {
> -		if (retry) {
> -			retry = false;
> +		if (!(flags & MF_NO_RETRY)) {
> +			flags |= MF_NO_RETRY;
>  			goto retry;
>  		}
>  		action_result(pfn, MF_MSG_UNKNOWN, MF_IGNORED);