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

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Date:   Thu, 3 Dec 2020 18:48:35 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Muchun Song <songmuchun@...edance.com>,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] mm/page_isolation: do not isolate the max order page

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@...edance.com>
>>>
>>> Acked-by: Vlastimil Babka <vbabka@...e.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