linux-kernel - Re: [PATCH 3/3] mm/memory-failure: send SIGBUS in the event of thp split fail

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Date: Mon, 6 May 2024 13:26:19 -0700
From: Jane Chu <jane.chu@...cle.com>
To: Miaohe Lin <linmiaohe@...wei.com>, nao.horiguchi@...il.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm/memory-failure: send SIGBUS in the event of thp
 split fail

On 5/5/2024 12:00 AM, Miaohe Lin wrote:

> On 2024/5/2 7:24, Jane Chu wrote:
>> When handle hwpoison in a GUP longterm pin'ed 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.
>>
>> Signed-off-by: Jane Chu <jane.chu@...cle.com>
> Thanks for your patch. Some comments below.
>
>> ---
>>   mm/memory-failure.c | 36 ++++++++++++++++++++++++++++++++++++
>>   1 file changed, 36 insertions(+)
>>
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 7fcf182abb96..67f4d24a98e7 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -2168,6 +2168,37 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>>   	return rc;
>>   }
>>   
>> +/*
>> + * The calling condition is as such: thp split failed, page might have
>> + * been GUP longterm pinned, not much can be done for recovery.
>> + * But a SIGBUS should be delivered with vaddr provided so that the user
>> + * application has a chance to recover. Also, application processes'
>> + * election for MCE early killed will be honored.
>> + */
>> +static int kill_procs_now(struct page *p, unsigned long pfn, int flags,
>> +			struct page *hpage)
>> +{
>> +	struct folio *folio = page_folio(hpage);
>> +	LIST_HEAD(tokill);
>> +	int res = -EHWPOISON;
>> +
>> +	/* deal with user pages only */
>> +	if (PageReserved(p) || PageSlab(p) || PageTable(p) || PageOffline(p))
>> +		res = -EBUSY;
>> +	if (!(PageLRU(hpage) || PageHuge(p)))
>> +		res = -EBUSY;
> Above checks seems unneeded. We already know it's thp?

Agreed.

I  lifted these checks from hwpoison_user_mapping() with a hope to make 
kill_procs_now() more generic,

such as, potentially replacing kill_accessing_processes() for 
re-accessing hwpoisoned page.

But I backed out at last, due to concerns that my tests might not have 
covered sufficient number of scenarios.

>
>> +
>> +	if (res == -EHWPOISON) {
>> +		collect_procs(folio, p, &tokill, flags & MF_ACTION_REQUIRED);
>> +		kill_procs(&tokill, true, pfn, flags);
>> +	}
>> +
>> +	if (flags & MF_COUNT_INCREASED)
>> +		put_page(p);
> This if block is broken. put_page() has been done when try_to_split_thp_page() fails?

put_page() has not been done if try_to_split_thp_page() fails, and I 
think it should.

I will revise the code so that put_page() is called regardless 
MF_ACTION_REQUIRED is set or not.

>
>> +
> action_result is missing?

Indeed,  action_result() isn't always called, referring to the 
re-accessing hwpoison scenarios.

In this case, I think the reason  is that, we just killed the process 
and there is nothing

else to do or to report.

>
>> +	return res;
>> +}
>> +
>>   /**
>>    * memory_failure - Handle memory failure of a page.
>>    * @pfn: Page Number of the corrupted page
>> @@ -2297,6 +2328,11 @@ int memory_failure(unsigned long pfn, int flags)
>>   		 */
>>   		SetPageHasHWPoisoned(hpage);
>>   		if (try_to_split_thp_page(p) < 0) {
> Should hwpoison_filter() be called in this case?
Yes, it should. I will add the hwpoison_filter check.
>
>> +			if (flags & MF_ACTION_REQUIRED) {
>> +				pr_err("%#lx: thp split failed\n", pfn);
>> +				res = kill_procs_now(p, pfn, flags, hpage);
> Can we use hwpoison_user_mappings() directly here?

I thought about using hwpoison_user_mappings() with an extra flag, but 
gave up in the end.

Because most of the code there are not needed, such as the checks you 
mentioned above,

and umapping etc.

thanks!

-jane

>
> Thanks.
> .
>
>> +				goto unlock_mutex;
>> +			}
>>   			res = action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
>>   			goto unlock_mutex;
>>   		}
>>