[PATCH] mm/page_isolation: do not isolate the max order page

[PATCH] mm/page_isolation: do not isolate the max order page

[Muchun Song]

The max order page has no buddy page and never merge to other order.
So isolating and then freeing it is pointless.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/page_isolation.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index a254e1f370a3..bddf788f45bf 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 	 */
 	if (PageBuddy(page)) {
 		order = buddy_order(page);
-		if (order >= pageblock_order) {
+		if (order >= pageblock_order && order < MAX_ORDER - 1) {
 			pfn = page_to_pfn(page);
 			buddy_pfn = __find_buddy_pfn(pfn, order);
 			buddy = page + (buddy_pfn - pfn);
-- 
2.11.0

Re: [PATCH] mm/page_isolation: do not isolate the max order page

[Vlastimil Babka]

On 12/2/20 1:21 PM, Muchun Song wrote:
> The max order page has no buddy page and never merge to other order.
> So isolating and then freeing it is pointless.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_isolation.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index a254e1f370a3..bddf788f45bf 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>  	 */
>  	if (PageBuddy(page)) {
>  		order = buddy_order(page);
> -		if (order >= pageblock_order) {
> +		if (order >= pageblock_order && order < MAX_ORDER - 1) {
>  			pfn = page_to_pfn(page);
>  			buddy_pfn = __find_buddy_pfn(pfn, order);
>  			buddy = page + (buddy_pfn - pfn);

Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
!pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
might access an invalid buddy. So this might be actually a bug fix and not just
optimization, just the bug hasn't been observed in practice.

> 

Re: [External] Re: [PATCH] mm/page_isolation: do not isolate the max order page

[Muchun Song]

On Thu, Dec 3, 2020 at 8:03 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 12/2/20 1:21 PM, Muchun Song wrote:
> > The max order page has no buddy page and never merge to other order.
> > So isolating and then freeing it is pointless.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>
> > ---
> >  mm/page_isolation.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > index a254e1f370a3..bddf788f45bf 100644
> > --- a/mm/page_isolation.c
> > +++ b/mm/page_isolation.c
> > @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
> >        */
> >       if (PageBuddy(page)) {
> >               order = buddy_order(page);
> > -             if (order >= pageblock_order) {
> > +             if (order >= pageblock_order && order < MAX_ORDER - 1) {
> >                       pfn = page_to_pfn(page);
> >                       buddy_pfn = __find_buddy_pfn(pfn, order);
> >                       buddy = page + (buddy_pfn - pfn);
>
> Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
> !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
> only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
> might access an invalid buddy. So this might be actually a bug fix and not just
> optimization, just the bug hasn't been observed in practice.

Agree. Should we add a Fixes tag in the commit log? Thanks.

>
> >
>


-- 
Yours,
Muchun

Re: [External] Re: [PATCH] mm/page_isolation: do not isolate the max order page

[Vlastimil Babka]

On 12/3/20 3:43 AM, Muchun Song wrote:
> On Thu, Dec 3, 2020 at 8:03 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> On 12/2/20 1:21 PM, Muchun Song wrote:
>> > The max order page has no buddy page and never merge to other order.
>> > So isolating and then freeing it is pointless.
>> >
>> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>
>> > ---
>> >  mm/page_isolation.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> > index a254e1f370a3..bddf788f45bf 100644
>> > --- a/mm/page_isolation.c
>> > +++ b/mm/page_isolation.c
>> > @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>> >        */
>> >       if (PageBuddy(page)) {
>> >               order = buddy_order(page);
>> > -             if (order >= pageblock_order) {
>> > +             if (order >= pageblock_order && order < MAX_ORDER - 1) {
>> >                       pfn = page_to_pfn(page);
>> >                       buddy_pfn = __find_buddy_pfn(pfn, order);
>> >                       buddy = page + (buddy_pfn - pfn);
>>
>> Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
>> !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
>> only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
>> might access an invalid buddy. So this might be actually a bug fix and not just
>> optimization, just the bug hasn't been observed in practice.
> 
> Agree. Should we add a Fixes tag in the commit log? Thanks.

Right.

Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated
pageblock")

The criteria for CC stable is not met though as it's theoretical.

>>
>> >
>>
> 
> 

Re: [PATCH] mm/page_isolation: do not isolate the max order page

[David Hildenbrand]

On 03.12.20 01:03, Vlastimil Babka wrote:
> On 12/2/20 1:21 PM, Muchun Song wrote:
>> The max order page has no buddy page and never merge to other order.
>> So isolating and then freeing it is pointless.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 
>> ---
>>  mm/page_isolation.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> index a254e1f370a3..bddf788f45bf 100644
>> --- a/mm/page_isolation.c
>> +++ b/mm/page_isolation.c
>> @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>  	 */
>>  	if (PageBuddy(page)) {
>>  		order = buddy_order(page);
>> -		if (order >= pageblock_order) {
>> +		if (order >= pageblock_order && order < MAX_ORDER - 1) {
>>  			pfn = page_to_pfn(page);
>>  			buddy_pfn = __find_buddy_pfn(pfn, order);
>>  			buddy = page + (buddy_pfn - pfn);
> 
> Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
> !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
> only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
> might access an invalid buddy. So this might be actually a bug fix and not just
> optimization, just the bug hasn't been observed in practice.

I think we have no users that isolate/unisolate close to holes.

CMA regions are properly aligned (to max of page_order /
max_order_nr_pages) and don't contain holes.

virtio-mem does not apply as it knows its range has no holes.

gigantic pages are aligned naturally and we check that there are no
holes upfront.

There are no other users.


I don't see a need for stable/fixes.

-- 
Thanks,

David / dhildenb

Re: [PATCH] mm/page_isolation: do not isolate the max order page

[David Hildenbrand]

On 02.12.20 13:21, Muchun Song wrote:
> The max order page has no buddy page and never merge to other order.
> So isolating and then freeing it is pointless.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/page_isolation.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index a254e1f370a3..bddf788f45bf 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>  	 */
>  	if (PageBuddy(page)) {
>  		order = buddy_order(page);
> -		if (order >= pageblock_order) {
> +		if (order >= pageblock_order && order < MAX_ORDER - 1) {
>  			pfn = page_to_pfn(page);
>  			buddy_pfn = __find_buddy_pfn(pfn, order);
>  			buddy = page + (buddy_pfn - pfn);
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb

Re: [PATCH] mm/page_isolation: do not isolate the max order page

[Vlastimil Babka]

On 12/3/20 5:26 PM, David Hildenbrand wrote:
> On 03.12.20 01:03, Vlastimil Babka wrote:
>> On 12/2/20 1:21 PM, Muchun Song wrote:
>>> The max order page has no buddy page and never merge to other order.
>>> So isolating and then freeing it is pointless.
>>>
>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> 
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>> 
>>> ---
>>>  mm/page_isolation.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>> index a254e1f370a3..bddf788f45bf 100644
>>> --- a/mm/page_isolation.c
>>> +++ b/mm/page_isolation.c
>>> @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>>  	 */
>>>  	if (PageBuddy(page)) {
>>>  		order = buddy_order(page);
>>> -		if (order >= pageblock_order) {
>>> +		if (order >= pageblock_order && order < MAX_ORDER - 1) {
>>>  			pfn = page_to_pfn(page);
>>>  			buddy_pfn = __find_buddy_pfn(pfn, order);
>>>  			buddy = page + (buddy_pfn - pfn);
>> 
>> Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
>> !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
>> only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
>> might access an invalid buddy. So this might be actually a bug fix and not just
>> optimization, just the bug hasn't been observed in practice.
> 
> I think we have no users that isolate/unisolate close to holes.
> 
> CMA regions are properly aligned (to max of page_order /
> max_order_nr_pages) and don't contain holes.

The problem as I see it, is that buddy_order(page) might be already MAX_ORDER -
1 (e.g. two pageblocks on x86), and then finding buddy of that one is beyond the
guaranteed alignment (if they merged, which they can't, it would be four
pageblocks). Might not be just a hole within zone, but also across zone boundary?
While being isolated and used pages migrated away, the freed pages shouldn't
merge to MAX_ORDER-1, but if the MAX_ORDER-1 free page was already there before
the isolation?

> virtio-mem does not apply as it knows its range has no holes.
> 
> gigantic pages are aligned naturally and we check that there are no
> holes upfront.
> 
> There are no other users.
> 
> 
> I don't see a need for stable/fixes.
> 

Re: [PATCH] mm/page_isolation: do not isolate the max order page

[David Hildenbrand]

On 03.12.20 18:15, Vlastimil Babka wrote:
> On 12/3/20 5:26 PM, David Hildenbrand wrote:
>> On 03.12.20 01:03, Vlastimil Babka wrote:
>>> On 12/2/20 1:21 PM, Muchun Song wrote:
>>>> The max order page has no buddy page and never merge to other order.
>>>> So isolating and then freeing it is pointless.
>>>>
>>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>>
>>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>>
>>>> ---
>>>>  mm/page_isolation.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>>> index a254e1f370a3..bddf788f45bf 100644
>>>> --- a/mm/page_isolation.c
>>>> +++ b/mm/page_isolation.c
>>>> @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>>>  	 */
>>>>  	if (PageBuddy(page)) {
>>>>  		order = buddy_order(page);
>>>> -		if (order >= pageblock_order) {
>>>> +		if (order >= pageblock_order && order < MAX_ORDER - 1) {
>>>>  			pfn = page_to_pfn(page);
>>>>  			buddy_pfn = __find_buddy_pfn(pfn, order);
>>>>  			buddy = page + (buddy_pfn - pfn);
>>>
>>> Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
>>> !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
>>> only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
>>> might access an invalid buddy. So this might be actually a bug fix and not just
>>> optimization, just the bug hasn't been observed in practice.
>>
>> I think we have no users that isolate/unisolate close to holes.
>>
>> CMA regions are properly aligned (to max of page_order /
>> max_order_nr_pages) and don't contain holes.
> 
> The problem as I see it, is that buddy_order(page) might be already MAX_ORDER -
> 1 (e.g. two pageblocks on x86), and then finding buddy of that one is beyond the
> guaranteed alignment (if they merged, which they can't, it would be four

Oh, I see. I would have assume that __find_buddy_pfn() would not hand
out invalid buddies. But you're right, it's generic:

pfn = 1024 (4M)
order = MAX_ORDER - 1 = 10
buddy_pfn = __find_buddy_pfn(pfn, order)

-> pfn ^ (1 << order) = 0


If that page has no struct page (!pfn_valid), we're doomed, I agree. It
would be problematic if we have alloc_contig_range() users with ranges
not aligned/multiples of to 8 MB (MAX_ORDER) I guess. virtio-mem and
gigantic pages should be fine. CMA might be problematic, though? Do we
have such small CMA ranges or with such alignment? COuld be I guess.

cma_init_reserved_mem() only checks

alignment = PAGE_SIZE << max_t(unsigned long, MAX_ORDER - 1,
pageblock_order);

> pageblocks). Might not be just a hole within zone, but also across zone boundary?
> While being isolated and used pages migrated away, the freed pages shouldn't
> merge to MAX_ORDER-1, but if the MAX_ORDER-1 free page was already there before
> the isolation?




-- 
Thanks,

David / dhildenb

Re: [PATCH] mm/page_isolation: do not isolate the max order page

[David Hildenbrand]

On 03.12.20 18:48, David Hildenbrand wrote:
> On 03.12.20 18:15, Vlastimil Babka wrote:
>> On 12/3/20 5:26 PM, David Hildenbrand wrote:
>>> On 03.12.20 01:03, Vlastimil Babka wrote:
>>>> On 12/2/20 1:21 PM, Muchun Song wrote:
>>>>> The max order page has no buddy page and never merge to other order.
>>>>> So isolating and then freeing it is pointless.
>>>>>
>>>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>>>
>>>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>>>
>>>>> ---
>>>>>  mm/page_isolation.c | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>>>> index a254e1f370a3..bddf788f45bf 100644
>>>>> --- a/mm/page_isolation.c
>>>>> +++ b/mm/page_isolation.c
>>>>> @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>>>>  	 */
>>>>>  	if (PageBuddy(page)) {
>>>>>  		order = buddy_order(page);
>>>>> -		if (order >= pageblock_order) {
>>>>> +		if (order >= pageblock_order && order < MAX_ORDER - 1) {
>>>>>  			pfn = page_to_pfn(page);
>>>>>  			buddy_pfn = __find_buddy_pfn(pfn, order);
>>>>>  			buddy = page + (buddy_pfn - pfn);
>>>>
>>>> Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
>>>> !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
>>>> only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
>>>> might access an invalid buddy. So this might be actually a bug fix and not just
>>>> optimization, just the bug hasn't been observed in practice.
>>>
>>> I think we have no users that isolate/unisolate close to holes.
>>>
>>> CMA regions are properly aligned (to max of page_order /
>>> max_order_nr_pages) and don't contain holes.
>>
>> The problem as I see it, is that buddy_order(page) might be already MAX_ORDER -
>> 1 (e.g. two pageblocks on x86), and then finding buddy of that one is beyond the
>> guaranteed alignment (if they merged, which they can't, it would be four
> 
> Oh, I see. I would have assume that __find_buddy_pfn() would not hand
> out invalid buddies. But you're right, it's generic:
> 
> pfn = 1024 (4M)
> order = MAX_ORDER - 1 = 10
> buddy_pfn = __find_buddy_pfn(pfn, order)
> 
> -> pfn ^ (1 << order) = 0
> 
> 
> If that page has no struct page (!pfn_valid), we're doomed, I agree. It
> would be problematic if we have alloc_contig_range() users with ranges
> not aligned/multiples of to 8 MB (MAX_ORDER) I guess. virtio-mem and
> gigantic pages should be fine. CMA might be problematic, though? Do we
> have such small CMA ranges or with such alignment? COuld be I guess.
> 
> cma_init_reserved_mem() only checks
> 
> alignment = PAGE_SIZE << max_t(unsigned long, MAX_ORDER - 1,
> pageblock_order);
> 

Thinking again (SPARSE), we always end up in a single memory section.
Usually, all pfns within a single section are valid. The only exception
is with HAVE_ARCH_PFN_VALID - arm and arm6.

arm64 also has HOLES_IN_ZONE - so we always check for pfn_valid() in
this code.

arm only has HAVE_ARCH_PFN_VALID with SPARSE on ARCH_OMAP1. So only in
that combination, we might run into that issue if I am not wrong.


Not sure about !SPARSE and mips.

-- 
Thanks,

David / dhildenb