Re: [PATCH 1/2] mm: compaction: consider the number of scanning compound pages in isolate fail path

[Vlastimil Babka]

On 3/13/23 11:37, Baolin Wang wrote:
> The commit b717d6b93b54 ("mm: compaction: include compound page count
> for scanning in pageblock isolation") had added compound page statistics
> for scanning in pageblock isolation, to make sure the number of scanned
> pages are always larger than the number of isolated pages when isolating
> mirgratable or free pageblock.
> 
> However, when failed to isolate the pages when scanning the mirgratable or
> free pageblock, the isolation failure path did not consider the scanning
> statistics of the compound pages, which can show the incorrect number of
> scanned pages in tracepoints or the vmstats to make people confusing about
> the page scanning pressure in memory compaction.
> 
> Thus we should take into account the number of scanning pages when failed
> to isolate the compound pages to make the statistics accurate.
> 
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---
>  mm/compaction.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 5a9501e0ae01..c9d9ad958e2a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -587,6 +587,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  				blockpfn += (1UL << order) - 1;
>  				cursor += (1UL << order) - 1;
>  			}
> +			nr_scanned += (1UL << order) - 1;

I'd rather put it in the block above that tests order < MAX_ORDER. Otherwise
as the comments say, the value can be bogus as it's racy.

>  			goto isolate_fail;
>  		}
>  
> @@ -873,9 +874,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  			cond_resched();
>  		}
>  
> -		nr_scanned++;
> -
>  		page = pfn_to_page(low_pfn);
> +		nr_scanned += compound_nr(page);

For the same reason, I'd rather leave the nr_scanned adjustment by order in
the specific code blocks where we know/think we have a compound or huge page
and have sanity checked the order/nr_pages, and not add an unchecked
compound_nr() here.

Thanks.

>  
>  		/*
>  		 * Check if the pageblock has already been marked skipped.
> @@ -1077,6 +1077,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  			 */
>  			if (unlikely(PageCompound(page) && !cc->alloc_contig)) {
>  				low_pfn += compound_nr(page) - 1;
> +				nr_scanned += compound_nr(page) - 1;
>  				SetPageLRU(page);
>  				goto isolate_fail_put;
>  			}
> @@ -1097,7 +1098,6 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  isolate_success_no_list:
>  		cc->nr_migratepages += compound_nr(page);
>  		nr_isolated += compound_nr(page);
> -		nr_scanned += compound_nr(page) - 1;
>  
>  		/*
>  		 * Avoid isolating too much unless this block is being