Re: [PATCH v2 1/3] mm/memory-failure.c: fix race with changing page compound again

[Mike Kravetz]

On 3/11/22 23:46, Miaohe Lin wrote:
> There is a race window where we got the compound_head, the hugetlb page
> could be freed to buddy, or even changed to another compound page just
> before we try to get hwpoison page. Think about the below race window:
>   CPU 1					  CPU 2
>   memory_failure_hugetlb
>   struct page *head = compound_head(p);
> 					  hugetlb page might be freed to
> 					  buddy, or even changed to another
> 					  compound page.
> 
>   get_hwpoison_page -- page is not what we want now...
> 
> If this race happens, just bail out. Also MF_MSG_DIFFERENT_PAGE_SIZE is
> introduced to record this event.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
>  include/linux/mm.h      |  1 +
>  include/ras/ras_event.h |  1 +
>  mm/memory-failure.c     | 12 ++++++++++++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c9bada4096ac..ef98cff2b253 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3253,6 +3253,7 @@ enum mf_action_page_type {
>  	MF_MSG_BUDDY,
>  	MF_MSG_DAX,
>  	MF_MSG_UNSPLIT_THP,
> +	MF_MSG_DIFFERENT_PAGE_SIZE,
>  	MF_MSG_UNKNOWN,
>  };
>  
> diff --git a/include/ras/ras_event.h b/include/ras/ras_event.h
> index d0337a41141c..1e694fd239b9 100644
> --- a/include/ras/ras_event.h
> +++ b/include/ras/ras_event.h
> @@ -374,6 +374,7 @@ TRACE_EVENT(aer_event,
>  	EM ( MF_MSG_BUDDY, "free buddy page" )				\
>  	EM ( MF_MSG_DAX, "dax page" )					\
>  	EM ( MF_MSG_UNSPLIT_THP, "unsplit thp" )			\
> +	EM ( MF_MSG_DIFFERENT_PAGE_SIZE, "different page size" )	\
>  	EMe ( MF_MSG_UNKNOWN, "unknown page" )
>  
>  /*
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5444a8ef4867..dabecd87ad3f 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -733,6 +733,7 @@ static const char * const action_page_types[] = {
>  	[MF_MSG_BUDDY]			= "free buddy page",
>  	[MF_MSG_DAX]			= "dax page",
>  	[MF_MSG_UNSPLIT_THP]		= "unsplit thp",
> +	[MF_MSG_DIFFERENT_PAGE_SIZE]	= "different page size",
>  	[MF_MSG_UNKNOWN]		= "unknown page",
>  };
>  
> @@ -1534,6 +1535,17 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
>  	}
>  
>  	lock_page(head);
> +
> +	/**
> +	 * The page could have changed compound pages due to race window.
> +	 * If this happens just bail out.
> +	 */
> +	if (!PageHuge(p) || compound_head(p) != head) {
> +		action_result(pfn, MF_MSG_DIFFERENT_PAGE_SIZE, MF_IGNORED);
> +		res = -EBUSY;

We have discussed this race in other versions of the patch.  When we encounter
the race, we have likely marked poison on the wrong page.  Correct?

Instead of printing a "different page size", would it be better to perhaps:
- Print a message that wrong page may be marked for poison?
- Clear the poison flag in the "head page" previously set?

-- 
Mike Kravetz

> +		goto out;
> +	}
> +
>  	page_flags = head->flags;
>  
>  	if (hwpoison_filter(p)) {