Re: [PATCH v6 2/4] mm/compaction: enable compacting >0 order folios.

[David Hildenbrand <david@redhat.com>]

On 16.02.24 18:04, 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.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Suggested-by: Huang Ying <ying.huang@intel.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Tested-by: Yu Zhao <yuzhao@google.com>
> Cc: Adam Manzanares <a.manzanares@samsung.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Kemeng Shi <shikemeng@huaweicloud.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Luis Chamberlain <mcgrof@kernel.org>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Yin Fengwei <fengwei.yin@intel.com>
> ---
>   mm/compaction.c | 66 ++++++++++++++++++++++++++++++++++++++-----------
>   1 file changed, 52 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index cc801ce099b4..aa6aad805c4d 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;
>   }
>   
> +/*


Can't you add these comments to the respective checks? Like

static bool skip_isolation_on_order(int order, int target_order)
{
	/*
	 * Unless we are performing global compaction (targert_order <
	 * 0), skip any folios that are larger than the target order: we
	 * wouldn't be here if we'd have a free folio with the desired
	 * target_order, so migrating this folio would likely fail
	 * later.
	 */
	if (target_order != -1 && order >= target_order)
		return true;
	/*
	 * We limit memory compaction to pageblocks and won't try
	 * creating free blocks of memory that are larger than that.
	 */
	return order >= pageblock_order;
}

Then, add a simple expressive function documentation (if really 
required) that doesn't contain all these details.

> + * 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,

I'm pretty sure you meant "reduce" ?

> + * 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
> @@ -947,7 +962,22 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   			valid_page = page;
>   		}
>   
> -		if (PageHuge(page) && cc->alloc_contig) {
> +		if (PageHuge(page)) {
> +			/*
> +			 * skip hugetlbfs if we are not compacting for pages
> +			 * bigger than its order. THPs and other compound pages
> +			 * are handled below.
> +			 */
> +			if (!cc->alloc_contig) {
> +				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;
> +			}
> +			/* for alloc_contig case */
>   			if (locked) {
>   				unlock_page_lruvec_irqrestore(locked, flags);
>   				locked = NULL;
> @@ -1008,21 +1038,24 @@ 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).
> -		 * 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.
> +		 * Regardless of being on LRU, compound pages such as THP
> +		 * (hugetlbfs is handled above) are not to be compacted unless
> +		 * we are attempting 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.
>   		 */
>   		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. */
> +			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;
>   			}
> -			goto isolate_fail;
>   		}
>   
>   		/*
> @@ -1165,10 +1198,11 @@ 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);
> @@ -1788,6 +1822,10 @@ 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;

Why the "> 0 " check ? Either it's large or it isn't.

Apart from that LGTM, but I am no compaction expert.

-- 
Cheers,

David / dhildenb