Re: [PATCH 01/12] mm/ksm: catch tail page abnormal in page_stable_node

[Alex Shi]

On 3/18/24 8:25 PM, David Hildenbrand wrote:
> On 18.03.24 13:14, alexs@xxxxxxxxxx wrote:
>> From: Alex Shi <alexs@xxxxxxxxxx>
>>
>> commit 19138349ed59 ("mm/migrate: Add folio_migrate_flags()") change the
>> meaning of func page_stable_node() to check the compound head for tail
>> 'page' instead of tail page self.
>> But seems both semantics are same at results, the func always return NULL
>>   for tail page. So adding a bug monitor here in case of abnormal.
>>
>> Signed-off-by: Alex Shi <alexs@xxxxxxxxxx>
>> Cc: Izik Eidus <izik.eidus@xxxxxxxxxxxxxxxxxx>
>> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>> Cc: Chris Wright <chrisw@xxxxxxxxxxxx>
>> To: linux-kernel@xxxxxxxxxxxxxxx
>> To: linux-mm@xxxxxxxxx
>> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> ---
>>   mm/ksm.c | 7 ++++++-
>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/ksm.c b/mm/ksm.c
>> index 8c001819cf10..3ff469961927 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -1091,7 +1091,12 @@ static inline struct ksm_stable_node *folio_stable_node(struct folio *folio)
>>     static inline struct ksm_stable_node *page_stable_node(struct page *page)
>>   {
>> -    return folio_stable_node(page_folio(page));
>> +    struct ksm_stable_node *node;
>> +
>> +    node = folio_stable_node(page_folio(page));
>> +    VM_BUG_ON_PAGE(PageTail(page) && node, page);
> 
> I don't really understand why we would want this.
> 
> Only KSM folios can have a node in the stable tree. KSM folios cannot be large folios. At that is precisely what folio_stable_node() checks.
> 
> If we'd have a large folio identify as a KSM folio we'd be in much bigger trouble.
> 
> 
> Besides, I'm sure you read "22) Do not crash the kernel" in Documentation/process/coding-style.rst
> 

Hi David, 

Thanks for comments!
Forgive my stupidity, I understand KSM stable tree has no compound pages, but when searching a tail page in ksm_do_scan(), why we couldn't be in a race, that another VM doing THP collapse on the same contents pages, while the 3rd vm is doing hugepage spliting?

Best regards!
Alex