[v1,2/6] mm/hwpoison: remove race consideration

Commit Message

Naoya Horiguchi June 14, 2021, 2:12 a.m. UTC

From: Naoya Horiguchi <[e-mail]>

Now memory_failure() and unpoison_memory() are protected by mf_mutex,
so no need to explicitly check races between them.  So remove them.

Signed-off-by: Naoya Horiguchi <[e-mail]>
---
 mm/memory-failure.c | 37 -------------------------------------
 1 file changed, 37 deletions(-)

Comments

Ding Hui June 15, 2021, 12:57 p.m. UTC | #1

On 2021/6/14 10:12, Naoya Horiguchi wrote:
> @@ -1956,17 +1938,6 @@ int unpoison_memory(unsigned long pfn)
>   		goto unlock_mutex;
>   	}
>   
> -	/*
> -	 * unpoison_memory() can encounter thp only when the thp is being
> -	 * worked by memory_failure() and the page lock is not held yet.
> -	 * In such case, we yield to memory_failure() and make unpoison fail.
> -	 */
> -	if (!PageHuge(page) && PageTransHuge(page)) {
> -		unpoison_pr_info("Unpoison: Memory failure is now running on %#lx\n",
> -				 pfn, &unpoison_rs);
> -		goto unlock_mutex;
> -	}
> -

if a huge page is in process of alloc or free, HUGETLB_PAGE_DTOR can be 
set after __SetPageHead() or be cleared before __ClearPageHead(), so 
this condition may be true in racy.

Do we need the racy test for this situation?

>   	if (!get_hwpoison_page(p, flags)) {
>   		if (TestClearPageHWPoison(p))
>   			num_poisoned_pages_dec();

HORIGUCHI NAOYA(堀口 直也) June 16, 2021, 12:11 a.m. UTC | #2

On Tue, Jun 15, 2021 at 08:57:06PM +0800, Ding Hui wrote:
> On 2021/6/14 10:12, Naoya Horiguchi wrote:
> > @@ -1956,17 +1938,6 @@ int unpoison_memory(unsigned long pfn)
> >   		goto unlock_mutex;
> >   	}
> > -	/*
> > -	 * unpoison_memory() can encounter thp only when the thp is being
> > -	 * worked by memory_failure() and the page lock is not held yet.
> > -	 * In such case, we yield to memory_failure() and make unpoison fail.
> > -	 */
> > -	if (!PageHuge(page) && PageTransHuge(page)) {
> > -		unpoison_pr_info("Unpoison: Memory failure is now running on %#lx\n",
> > -				 pfn, &unpoison_rs);
> > -		goto unlock_mutex;
> > -	}
> > -
> 
> if a huge page is in process of alloc or free, HUGETLB_PAGE_DTOR can be set
> after __SetPageHead() or be cleared before __ClearPageHead(), so this
> condition may be true in racy.

Hi Ding,

We confirm PageHWPoison() before reaching this if-block and hwpoisoned pages
are prohibited from allocation, so it seems to me that this check never
races with hugetlb allocation.

And according to the original patch introduced this if-block (0cea3fdc416d:
"mm/hwpoison: fix race against poison thp"), this if-block intended to close
the race between memory_failure() and unpoison_memory(), so that's no longer
necessary due to mf_mutex.

> Do we need the racy test for this situation?

I'm not sure, but I think that we need more stress/fuzz testing focusing on
this subsystem, and "unpoison vs allocation" race can be covered in the topic.

Thank you,
Naoya Horiguchi

Ding Hui June 16, 2021, 12:40 a.m. UTC | #3

On 2021/6/16 8:11, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Jun 15, 2021 at 08:57:06PM +0800, Ding Hui wrote:
>> On 2021/6/14 10:12, Naoya Horiguchi wrote:
>>> @@ -1956,17 +1938,6 @@ int unpoison_memory(unsigned long pfn)
>>>    		goto unlock_mutex;
>>>    	}
>>> -	/*
>>> -	 * unpoison_memory() can encounter thp only when the thp is being
>>> -	 * worked by memory_failure() and the page lock is not held yet.
>>> -	 * In such case, we yield to memory_failure() and make unpoison fail.
>>> -	 */
>>> -	if (!PageHuge(page) && PageTransHuge(page)) {
>>> -		unpoison_pr_info("Unpoison: Memory failure is now running on %#lx\n",
>>> -				 pfn, &unpoison_rs);
>>> -		goto unlock_mutex;
>>> -	}
>>> -
>>
>> if a huge page is in process of alloc or free, HUGETLB_PAGE_DTOR can be set
>> after __SetPageHead() or be cleared before __ClearPageHead(), so this
>> condition may be true in racy.
> 
> Hi Ding,
> 
> We confirm PageHWPoison() before reaching this if-block and hwpoisoned pages
> are prohibited from allocation, so it seems to me that this check never
> races with hugetlb allocation.
> 
> And according to the original patch introduced this if-block (0cea3fdc416d:
> "mm/hwpoison: fix race against poison thp"), this if-block intended to close
> the race between memory_failure() and unpoison_memory(), so that's no longer
> necessary due to mf_mutex.
> 

I got it and thanks for your explanation.

>> Do we need the racy test for this situation?
> 
> I'm not sure, but I think that we need more stress/fuzz testing focusing on
> this subsystem, and "unpoison vs allocation" race can be covered in the topic.
> 
> Thank you,
> Naoya Horiguchi
>

Patch

diff --git v5.13-rc5/mm/memory-failure.c v5.13-rc5_patched/mm/memory-failure.c
index 280eb6d6dd15..e7910386fc9c 100644
--- v5.13-rc5/mm/memory-failure.c
+++ v5.13-rc5_patched/mm/memory-failure.c
@@ -1461,14 +1461,6 @@  static int memory_failure_hugetlb(unsigned long pfn, int flags)
 	lock_page(head);
 	page_flags = head->flags;
 
-	if (!PageHWPoison(head)) {
-		pr_err("Memory failure: %#lx: just unpoisoned\n", pfn);
-		num_poisoned_pages_dec();
-		unlock_page(head);
-		put_page(head);
-		return 0;
-	}
-
 	/*
 	 * TODO: hwpoison for pud-sized hugetlb doesn't work right now, so
 	 * simply disable it. In order to make it work properly, we need
@@ -1730,16 +1722,6 @@  int memory_failure(unsigned long pfn, int flags)
 	 */
 	page_flags = p->flags;
 
-	/*
-	 * unpoison always clear PG_hwpoison inside page lock
-	 */
-	if (!PageHWPoison(p)) {
-		pr_err("Memory failure: %#lx: just unpoisoned\n", pfn);
-		num_poisoned_pages_dec();
-		unlock_page(p);
-		put_page(p);
-		goto unlock_mutex;
-	}
 	if (hwpoison_filter(p)) {
 		if (TestClearPageHWPoison(p))
 			num_poisoned_pages_dec();
@@ -1956,17 +1938,6 @@  int unpoison_memory(unsigned long pfn)
 		goto unlock_mutex;
 	}
 
-	/*
-	 * unpoison_memory() can encounter thp only when the thp is being
-	 * worked by memory_failure() and the page lock is not held yet.
-	 * In such case, we yield to memory_failure() and make unpoison fail.
-	 */
-	if (!PageHuge(page) && PageTransHuge(page)) {
-		unpoison_pr_info("Unpoison: Memory failure is now running on %#lx\n",
-				 pfn, &unpoison_rs);
-		goto unlock_mutex;
-	}
-
 	if (!get_hwpoison_page(p, flags)) {
 		if (TestClearPageHWPoison(p))
 			num_poisoned_pages_dec();
@@ -1975,20 +1946,12 @@  int unpoison_memory(unsigned long pfn)
 		goto unlock_mutex;
 	}
 
-	lock_page(page);
-	/*
-	 * This test is racy because PG_hwpoison is set outside of page lock.
-	 * That's acceptable because that won't trigger kernel panic. Instead,
-	 * the PG_hwpoison page will be caught and isolated on the entrance to
-	 * the free buddy page pool.
-	 */
 	if (TestClearPageHWPoison(page)) {
 		unpoison_pr_info("Unpoison: Software-unpoisoned page %#lx\n",
 				 pfn, &unpoison_rs);
 		num_poisoned_pages_dec();
 		freeit = 1;
 	}
-	unlock_page(page);
 
 	put_page(page);
 	if (freeit && !(pfn == my_zero_pfn(0) && page_count(p) == 1))