Re: [PATCH v2 4/8] mm/memory-failure.c: fix race with changing page more robustly

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Wed, Feb 16, 2022 at 05:14:27PM +0800, Miaohe Lin wrote:
> We're only intended to deal with the non-Compound page after we split thp
> in memory_failure. However, the page could have changed compound pages due
> to race window. If this happens, we could try again to hopefully handle the
> page next round. Also remove unneeded orig_head. It's always equal to the
> hpage. So we can use hpage directly and remove this redundant one.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  mm/memory-failure.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 7e205d91b2d7..d66f642888be 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1690,7 +1690,6 @@ int memory_failure(unsigned long pfn, int flags)
>  {
>  	struct page *p;
>  	struct page *hpage;
> -	struct page *orig_head;
>  	struct dev_pagemap *pgmap;
>  	int res = 0;
>  	unsigned long page_flags;
> @@ -1736,7 +1735,7 @@ int memory_failure(unsigned long pfn, int flags)
>  		goto unlock_mutex;
>  	}
>  
> -	orig_head = hpage = compound_head(p);
> +	hpage = compound_head(p);
>  	num_poisoned_pages_inc();
>  
>  	/*
> @@ -1817,13 +1816,18 @@ int memory_failure(unsigned long pfn, int flags)
>  	lock_page(p);
>  
>  	/*
> -	 * The page could have changed compound pages during the locking.
> -	 * If this happens just bail out.
> +	 * We're only intended to deal with the non-Compound page here.
> +	 * However, the page could have changed compound pages due to
> +	 * race window. If this happens, we could try again to hopefully
> +	 * handle the page next round.
>  	 */
> -	if (PageCompound(p) && compound_head(p) != orig_head) {
> -		action_result(pfn, MF_MSG_DIFFERENT_COMPOUND, MF_IGNORED);
> -		res = -EBUSY;
> -		goto unlock_page;
> +	if (PageCompound(p)) {
> +		if (TestClearPageHWPoison(p))
> +			num_poisoned_pages_dec();
> +		unlock_page(p);
> +		put_page(p);
> +		flags &= ~MF_COUNT_INCREASED;

Could you limit the retry chance only once by using the local variable
"retry"?  It might be very rare to hit the race more than once in a single
error event, but just to be safe from potential infinite loop (that could be
opened by future changes).

Thanks,
Naoya Horiguchi

> +		goto try_again;
>  	}
>  
>  	/*
> -- 
> 2.23.0