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

[Zi Yan <ziy@nvidia.com>]

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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) {
        // 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;
    }
}

...

if (PageCompound(page) && !cc->alloc_contig) {
        const unsigned int order = compound_order(page);

        /* Skip based on page order and compaction target order. */
        if (skip_isolation_on_order(order, cc->order)) {
                if (order <= MAX_PAGE_ORDER) {
                        low_pfn += (1UL << order) - 1;
                        nr_scanned += (1UL << order) - 1;
                }
                goto isolate_fail;
        }
}

>
>> +				if (order <= MAX_PAGE_ORDER) {
>> +					low_pfn += (1UL << order) - 1;
>> +					nr_scanned += (1UL << order) - 1;
>> +				}
>> +				goto isolate_fail;
>>  			}
>> -			goto isolate_fail;
>>  		}
>>
>>  		/*
>> @@ -1165,10 +1187,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>  			}
>>
>>  			/*
>> -			 * folio become large since the non-locked check,
>> -			 * and it's on LRU.
>> +			 * Check LRU folio order under the lock
>>  			 */
>> -			if (unlikely(folio_test_large(folio) && !cc->alloc_contig)) {
>> +			if (unlikely(skip_isolation_on_order(folio_order(folio),
>> +							     cc->order) &&
>> +				     !cc->alloc_contig)) {
>>  				low_pfn += folio_nr_pages(folio) - 1;
>>  				nr_scanned += folio_nr_pages(folio) - 1;
>>  				folio_set_lru(folio);
>> @@ -1786,6 +1809,10 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>>  	struct compact_control *cc = (struct compact_control *)data;
>>  	struct folio *dst;
>>
>> +	/* this makes migrate_pages() split the source page and retry */
>> +	if (folio_test_large(src) > 0)
>> +		return NULL;
>> +
>>  	if (list_empty(&cc->freepages)) {
>>  		isolate_freepages(cc);
>>


--
Best Regards,
Yan, Zi

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