linux-kernel - Re: [PATCH v2 5/5] mm/memory-failure: send SIGBUS in the event of thp split fail

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Message-ID: <ZkYARVW2cOZcsFYB@localhost.localdomain>
Date: Thu, 16 May 2024 14:47:01 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Jane Chu <jane.chu@...cle.com>
Cc: linmiaohe@...wei.com, nao.horiguchi@...il.com,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] mm/memory-failure: send SIGBUS in the event of
 thp split fail

On Fri, May 10, 2024 at 12:26:02AM -0600, Jane Chu wrote:
> When handle hwpoison in a RDMA longterm pinned thp page,
> try_to_split_thp_page() will fail. And at this point, there is
> little else the kernel could do except sending a SIGBUS to
> the user process, thus give it a chance to recover.

Well, it does need to be a RDMA longterm pinned, right?
Anything holding an extra refcount can already make us bite the dust, so
I would not make it that specific.


> Signed-off-by: Jane Chu <jane.chu@...cle.com>
> ---
>  mm/memory-failure.c | 31 ++++++++++++++++++++++++++-----
>  1 file changed, 26 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 2fa884d8b5a3..15bb1c0c42e8 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1697,7 +1697,7 @@ static int identify_page_state(unsigned long pfn, struct page *p,
>  	return page_action(ps, p, pfn);
>  }
>  
> -static int try_to_split_thp_page(struct page *page)
> +static int try_to_split_thp_page(struct page *page, bool release)
>  {
>  	int ret;
>  
> @@ -1705,7 +1705,7 @@ static int try_to_split_thp_page(struct page *page)
>  	ret = split_huge_page(page);
>  	unlock_page(page);
>  
> -	if (unlikely(ret))
> +	if (ret && release)
>  		put_page(page);

I would document whhen and when not we can release the page.
E.g: we cannot release it if there are still processes mapping the thp.


> +static int kill_procs_now(struct page *p, unsigned long pfn, int flags,
> +				struct folio *folio)
> +{
> +	LIST_HEAD(tokill);
> +
> +	collect_procs(folio, p, &tokill, flags & MF_ACTION_REQUIRED);
> +	kill_procs(&tokill, true, pfn, flags);
> +
> +	return -EHWPOISON;

You are returning -EHWPOISON here,

> +}
> +
>  /**
>   * memory_failure - Handle memory failure of a page.
>   * @pfn: Page Number of the corrupted page
> @@ -2313,8 +2331,11 @@ int memory_failure(unsigned long pfn, int flags)
>  		 * page is a valid handlable page.
>  		 */
>  		folio_set_has_hwpoisoned(folio);
> -		if (try_to_split_thp_page(p) < 0) {
> -			res = action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
> +		if (try_to_split_thp_page(p, false) < 0) {
> +			pr_err("%#lx: thp split failed\n", pfn);
> +			res = kill_procs_now(p, pfn, flags, folio);
> +			put_page(p);
> +			res = action_result(pfn, MF_MSG_UNSPLIT_THP, MF_FAILED);

just to overwrite it here with action_result(). Which one do we need?
I think we would need -EBUSY here, right? So I would drop the retcode
from kill_procs_now.

Also, do we want the extra pr_err() here.
action_result() will already provide us the pfn and the
action_page_types which will be "unsplit thp". Is not that clear enough?

I would drop that.


-- 
Oscar Salvador
SUSE Labs