Re: [RFC PATCH] mm: compaction: avoid fast_isolate_freepages blindly choose improper pageblock

[Baolin Wang]

On 11/29/2023 6:45 PM, Barry Song wrote:
> Testing shows fast_isolate_freepages can blindly choose an unsuitable
> pageblock from time to time particularly while the min mark is used
> from XXX path:
>   if (!page) {
>           cc->fast_search_fail++;
>           if (scan_start) {
>                   /*
>                    * Use the highest PFN found above min. If one was
>                    * not found, be pessimistic for direct compaction
>                    * and use the min mark.
>                    */
>                   if (highest >= min_pfn) {
>                           page = pfn_to_page(highest);
>                           cc->free_pfn = highest;
>                   } else {
>                           if (cc->direct_compaction && pfn_valid(min_pfn)) { /* XXX */
>                                   page = pageblock_pfn_to_page(min_pfn,
>                                           min(pageblock_end_pfn(min_pfn),
>                                               zone_end_pfn(cc->zone)),
>                                           cc->zone);
>                                   cc->free_pfn = min_pfn;
>                           }
>                   }
>           }
>   }

Yes, the min_pfn can be an unsuitable migration target. But I think we 
can just add the suitable_migration_target() validation into 'min_pfn' 
case? Since other cases must be suitable target which found from 
MIGRATE_MOVABLE free list. Something like below:

diff --git a/mm/compaction.c b/mm/compaction.c
index 01ba298739dd..4e8eb4571909 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1611,6 +1611,8 @@ static void fast_isolate_freepages(struct 
compact_control *cc)

min(pageblock_end_pfn(min_pfn),

zone_end_pfn(cc->zone)),
                                                cc->zone);
+                                       if 
(!suitable_migration_target(cc, page))
+                                               page = NULL;
                                        cc->free_pfn = min_pfn;
                                }
                        }

By the way, I wonder if this patch can improve the efficiency of 
compaction in your test case?

> In contrast, slow path is skipping unsuitable pageblocks in a decent way.
>
> I don't know if it is an intended design or just an oversight. But
> it seems more sensible to skip unsuitable pageblock.
>
> Reported-by: Zhanyuan Hu <huzhanyuan@xxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
>   mm/compaction.c | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 01ba298739dd..98c485a25614 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1625,6 +1625,12 @@ static void fast_isolate_freepages(struct compact_control *cc)
>   	cc->total_free_scanned += nr_scanned;
>   	if (!page)
>   		return;
> +	/*
> +	 * Otherwise, we can blindly choose an improper pageblock especially
> +	 * while using the min mark
> +	 */
> +	if (!suitable_migration_target(cc, page))
> +		return;
>   
>   	low_pfn = page_to_pfn(page);
>   	fast_isolate_around(cc, low_pfn);