Re: [PATCH 4/8] mm/memory-failure.c: remove unneeded orig_head

[Miaohe Lin]

On 2022/2/14 22:50, Naoya Horiguchi wrote:
> On Thu, Feb 10, 2022 at 10:17:29PM +0800, Miaohe Lin wrote:
>> orig_head is used to check whether the page have changed compound pages
>> during the locking. But it's always equal to hpage. So we can use hpage
>> directly and remove this redundant one.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>>  mm/memory-failure.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 2dd7f35ee65a..4370c2f407c5 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1691,7 +1691,6 @@ 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;
>> @@ -1737,7 +1736,7 @@ 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();
>>
>>  	/*
>> @@ -1821,7 +1820,7 @@ int memory_failure(unsigned long pfn, int flags)
>>  	 * The page could have changed compound pages during the locking.
>>  	 * If this happens just bail out.
>>  	 */
>> -	if (PageCompound(p) && compound_head(p) != orig_head) {
>> +	if (PageCompound(p) && compound_head(p) != hpage) {
> 
> I think that this if-check was intended to detect the case that page p
> belongs to a thp when memory_failure() is called and belongs to a compound
> page in different size (like slab or some driver page) after the thp is
> split.  But your suggestion makes me aware that the page p could be embedded
> on a thp again after thp split.  I think this might be rare, but if it

IIUC, this page can't be embedded on a thp again after thp split because memory_failure hold
an __extra__ page refcnt. I think there exist below race windows:

memory_failure
  orig_head = hpage = compound_head(p); -- page is Non-compound yet
  < -- Page becomes compound page, like thp, slab, some driver page and even hugetlb page -- >
  get_hwpoison_page
  failed to split thp page, as hpage is Non-compound ...
  lock_page

> happens the current if-check (or suggested one) cannot detect it.
> So I feel that simply dropping compound_head() check might be better?
> 
> -	if (PageCompound(p) && compound_head(p) != orig_head) {
> +	if (PageCompound(p)) {

However this change could also catch the above race correctly. In fact, we can't handle
compound page here. But is it enough to just return -EBUSY here as it's really rare or
we should do more things (like split thp, retry if in PageHuge case)?

> 
> This should ensure the assumption (mentioned in 8/8 patch) more that
> the error page never belongs to compound page after taking page lock.

Agree, this really helps.

>> Thanks,
> Naoya Horiguchi
> .
> 

Many thanks.