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

[David Hildenbrand <david@redhat.com>]

On 20.03.24 10:05, Alex Shi wrote:
> 
> 
> On 3/18/24 8:25 PM, David Hildenbrand wrote:
>> On 18.03.24 13:14, alexs@kernel.org wrote:
>>> From: Alex Shi <alexs@kernel.org>
>>>
>>> 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@kernel.org>
>>> Cc: Izik Eidus <izik.eidus@ravellosystems.com>
>>> Cc: Matthew Wilcox <willy@infradead.org>
>>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>>> Cc: Hugh Dickins <hughd@google.com>
>>> Cc: Chris Wright <chrisw@sous-sol.org>
>>> To: linux-kernel@vger.kernel.org
>>> To: linux-mm@kvack.org
>>> To: Andrew Morton <akpm@linux-foundation.org>
>>> ---
>>>    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?

We always call cmp_and_merge_page() while holding a reference on the page.

There, we call page_stable_node() directly and via 
stable_tree_search()->page_stable_node() on that page.

When stable_tree_search() returns a kpage, we also hold a reference to 
that kpage. So calling page_stable_node() on the kpage behaves the same.

As we are holding page references, pages cannot be split/merged and we 
should not see any races in page_stable_node().

Am I missing something?

Note that your change would also not help here: if it would be racy, 
you'd also not reliably catch any tail pages.

But it should not be racy unless I am missing something.

-- 
Cheers,

David / dhildenb