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

Commit Message

[e-mail] March 18, 2024, 12:14 p.m. UTC

From: Alex Shi <[e-mail]>

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 <[e-mail]>
Cc: Izik Eidus <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Chris Wright <[e-mail]>
To: [e-mail]
To: [e-mail]
To: Andrew Morton <[e-mail]>
---
 mm/ksm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Comments

David Hildenbrand March 18, 2024, 12:25 p.m. UTC | #1

On 18.03.24 13:14, [e-mail] wrote:
> From: Alex Shi <[e-mail]>
> 
> 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 <[e-mail]>
> Cc: Izik Eidus <[e-mail]>
> Cc: Matthew Wilcox <[e-mail]>
> Cc: Andrea Arcangeli <[e-mail]>
> Cc: Hugh Dickins <[e-mail]>
> Cc: Chris Wright <[e-mail]>
> To: [e-mail]
> To: [e-mail]
> To: Andrew Morton <[e-mail]>
> ---
>   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

Matthew Wilcox March 18, 2024, 1:14 p.m. UTC | #2

On Mon, Mar 18, 2024 at 08:14:30PM +0800, [e-mail] wrote:
> From: Alex Shi <[e-mail]>
> 
> 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.

No.  I didn't object to you doing this to convince yourself, but there's
no way we should have this upstream.

If we did have large KSM folios (and I don't think we should!), you'd
crash the kernel instead of silently doing the right thing.

>  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);
> +
> +	return node;
>  }

Alex Shi March 20, 2024, 9:05 a.m. UTC | #3

On 3/18/24 8:25 PM, David Hildenbrand wrote:
> On 18.03.24 13:14, [e-mail] wrote:
>> From: Alex Shi <[e-mail]>
>>
>> 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 <[e-mail]>
>> Cc: Izik Eidus <[e-mail]>
>> Cc: Matthew Wilcox <[e-mail]>
>> Cc: Andrea Arcangeli <[e-mail]>
>> Cc: Hugh Dickins <[e-mail]>
>> Cc: Chris Wright <[e-mail]>
>> To: [e-mail]
>> To: [e-mail]
>> To: Andrew Morton <[e-mail]>
>> ---
>>   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

David Hildenbrand March 20, 2024, 9:29 a.m. UTC | #4

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, [e-mail] wrote:
>>> From: Alex Shi <[e-mail]>
>>>
>>> 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 <[e-mail]>
>>> Cc: Izik Eidus <[e-mail]>
>>> Cc: Matthew Wilcox <[e-mail]>
>>> Cc: Andrea Arcangeli <[e-mail]>
>>> Cc: Hugh Dickins <[e-mail]>
>>> Cc: Chris Wright <[e-mail]>
>>> To: [e-mail]
>>> To: [e-mail]
>>> To: Andrew Morton <[e-mail]>
>>> ---
>>>    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.

Alex Shi March 20, 2024, 12:03 p.m. UTC | #5

On 3/20/24 5:29 PM, David Hildenbrand wrote:
> 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, [e-mail] wrote:
>>>> From: Alex Shi <[e-mail]>
>>>>
>>>> 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 <[e-mail]>
>>>> Cc: Izik Eidus <[e-mail]>
>>>> Cc: Matthew Wilcox <[e-mail]>
>>>> Cc: Andrea Arcangeli <[e-mail]>
>>>> Cc: Hugh Dickins <[e-mail]>
>>>> Cc: Chris Wright <[e-mail]>
>>>> To: [e-mail]
>>>> To: [e-mail]
>>>> To: Andrew Morton <[e-mail]>
>>>> ---
>>>>    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.
> 

Hi David,

Thanks for the info, I see.

BTW, I should cc you for my KSM folio patchset review: https://lore.kernel.org/linux-mm/[e-mail]/

Best regards!
Alex

Patch

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);
+
+	return node;
 }
 
 static inline void set_page_stable_node(struct page *page,