From: Alex Shi <[email protected]>
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 <[email protected]>
Cc: Izik Eidus <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Chris Wright <[email protected]>
To: [email protected]
To: [email protected]
To: Andrew Morton <[email protected]>
---
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);
+
+ return node;
}
static inline void set_page_stable_node(struct page *page,
--
2.43.0
On 18.03.24 13:14, [email protected] wrote:
> From: Alex Shi <[email protected]>
>
> 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 <[email protected]>
> Cc: Izik Eidus <[email protected]>
> Cc: Matthew Wilcox <[email protected]>
> Cc: Andrea Arcangeli <[email protected]>
> Cc: Hugh Dickins <[email protected]>
> Cc: Chris Wright <[email protected]>
> To: [email protected]
> To: [email protected]
> To: Andrew Morton <[email protected]>
> ---
> 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
--
Cheers,
David / dhildenb
On Mon, Mar 18, 2024 at 08:14:30PM +0800, [email protected] wrote:
> From: Alex Shi <[email protected]>
>
> 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;
> }
On 3/18/24 8:25 PM, David Hildenbrand wrote:
> On 18.03.24 13:14, [email protected] wrote:
>> From: Alex Shi <[email protected]>
>>
>> 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 <[email protected]>
>> Cc: Izik Eidus <[email protected]>
>> Cc: Matthew Wilcox <[email protected]>
>> Cc: Andrea Arcangeli <[email protected]>
>> Cc: Hugh Dickins <[email protected]>
>> Cc: Chris Wright <[email protected]>
>> To: [email protected]
>> To: [email protected]
>> To: Andrew Morton <[email protected]>
>> ---
>> 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
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, [email protected] wrote:
>>> From: Alex Shi <[email protected]>
>>>
>>> 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 <[email protected]>
>>> Cc: Izik Eidus <[email protected]>
>>> Cc: Matthew Wilcox <[email protected]>
>>> Cc: Andrea Arcangeli <[email protected]>
>>> Cc: Hugh Dickins <[email protected]>
>>> Cc: Chris Wright <[email protected]>
>>> To: [email protected]
>>> To: [email protected]
>>> To: Andrew Morton <[email protected]>
>>> ---
>>> 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
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, [email protected] wrote:
>>>> From: Alex Shi <[email protected]>
>>>>
>>>> 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 <[email protected]>
>>>> Cc: Izik Eidus <[email protected]>
>>>> Cc: Matthew Wilcox <[email protected]>
>>>> Cc: Andrea Arcangeli <[email protected]>
>>>> Cc: Hugh Dickins <[email protected]>
>>>> Cc: Chris Wright <[email protected]>
>>>> To: [email protected]
>>>> To: [email protected]
>>>> To: Andrew Morton <[email protected]>
>>>> ---
>>>> 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/[email protected]/
Best regards!
Alex