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

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Message-ID: <b6c1b513-4470-4721-120c-1b1c813b2680@huawei.com>
Date: Wed, 8 May 2024 16:08:55 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: Jane Chu <jane.chu@...cle.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 2024/5/7 4:26, Jane Chu wrote:
> 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.

In try_to_split_thp_page(), if split_huge_page fails, i.e. ret != 0, put_page() is called. See below:

static int try_to_split_thp_page(struct page *page)
{
	int ret;

	lock_page(page);
	ret = split_huge_page(page);
	unlock_page(page);

	if (unlikely(ret))
		put_page(page);
	^^^^^^^^^^^^^^^^^^^^^^^
	return ret;
}

Or am I miss something?

> 
> 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) {

Only in MF_ACTION_REQUIRED case, SIGBUS is sent to processes when thp split failed. Any reson under it?

Thanks.
.