Re: [PATCH v3 1/3] mm/compaction: enable compacting >0 order folios.

[Vlastimil Babka <vbabka@suse.cz>]

On 2/9/24 20:25, Zi Yan wrote:
> On 9 Feb 2024, at 9:32, Vlastimil Babka wrote:
> 
>> On 2/2/24 17:15, Zi Yan wrote:
>>> From: Zi Yan <ziy@nvidia.com>
>>>
>>> migrate_pages() supports >0 order folio migration and during compaction,
>>> even if compaction_alloc() cannot provide >0 order free pages,
>>> migrate_pages() can split the source page and try to migrate the base pages
>>> from the split. It can be a baseline and start point for adding support for
>>> compacting >0 order folios.
>>>
>>> Suggested-by: Huang Ying <ying.huang@intel.com>
>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>> ---
>>>  mm/compaction.c | 43 +++++++++++++++++++++++++++++++++++--------
>>>  1 file changed, 35 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index 4add68d40e8d..e43e898d2c77 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -816,6 +816,21 @@ static bool too_many_isolated(struct compact_control *cc)
>>>  	return too_many;
>>>  }
>>>
>>> +/*
>>> + * 1. if the page order is larger than or equal to target_order (i.e.,
>>> + * cc->order and when it is not -1 for global compaction), skip it since
>>> + * target_order already indicates no free page with larger than target_order
>>> + * exists and later migrating it will most likely fail;
>>> + *
>>> + * 2. compacting > pageblock_order pages does not improve memory fragmentation,
>>> + * skip them;
>>> + */
>>> +static bool skip_isolation_on_order(int order, int target_order)
>>> +{
>>> +	return (target_order != -1 && order >= target_order) ||
>>> +		order >= pageblock_order;
>>> +}
>>> +
>>>  /**
>>>   * isolate_migratepages_block() - isolate all migrate-able pages within
>>>   *				  a single pageblock
>>> @@ -1010,7 +1025,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>>  		/*
>>>  		 * Regardless of being on LRU, compound pages such as THP and
>>>  		 * hugetlbfs are not to be compacted unless we are attempting
>>> -		 * an allocation much larger than the huge page size (eg CMA).
>>> +		 * an allocation larger than the compound page size.
>>>  		 * We can potentially save a lot of iterations if we skip them
>>>  		 * at once. The check is racy, but we can consider only valid
>>>  		 * values and the only danger is skipping too much.
>>> @@ -1018,11 +1033,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>>  		if (PageCompound(page) && !cc->alloc_contig) {
>>>  			const unsigned int order = compound_order(page);
>>>
>>> -			if (likely(order <= MAX_PAGE_ORDER)) {
>>> -				low_pfn += (1UL << order) - 1;
>>> -				nr_scanned += (1UL << order) - 1;
>>> +			/*
>>> +			 * Skip based on page order and compaction target order
>>> +			 * and skip hugetlbfs pages.
>>> +			 */
>>> +			if (skip_isolation_on_order(order, cc->order) ||
>>> +			    PageHuge(page)) {
>>
>> Hm I'd try to avoid a new PageHuge() test here.
>>
>> Earlier we have a block that does
>>                 if (PageHuge(page) && cc->alloc_contig) {
>> 			...
>>
>> think I'd rather rewrite it to handle the PageHuge() case completely and
>> just make it skip the 1UL << order pages there for !cc->alloc_config. Even
>> if it means duplicating a bit of the low_pfn and nr_scanned bumping code.
>>
>> Which reminds me the PageHuge() check there is probably still broken ATM:
>>
>> https://lore.kernel.org/all/8fa1c95c-4749-33dd-42ba-243e492ab109@suse.cz/
>>
>> Even better reason not to add another one.
>> If the huge page materialized since the first check, we should bail out when
>> testing PageLRU later anyway.
> 
> 
> OK, so basically something like:
> 
> if (PageHuge(page)) {
>     if (cc->alloc_contig) {

Yeah but I'd handle the !cc->alloc_contig first as that ends with a goto,
and then the rest doesn't need to be "} else { ... }" with extra identation

>         // existing code for PageHuge(page) && cc->allc_contig
>     } else {
>         const unsigned int order = compound_order(page);
> 
>         if (order <= MAX_PAGE_ORDER) {
>             low_pfn += (1UL << order) - 1;
>             nr_scanned += (1UL << order) - 1;
>         }
>         goto isolate_fail;
>     }
> }