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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 12/6/2023 7:00 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;
>                           }
>                   }
>           }
>   }
> 
> The reason is that no code is doing any check on the min_pfn
>   min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1));
> 
> In contrast, slow path of isolate_freepages() is always skipping unsuitable
> pageblocks in a decent way.
> 
> This issue doesn't happen quite often. When running 25 machines with 16GiB
> memory for one night, most of them can hit this unexpected code path.
> However the frequency isn't like many times per second. It might be one
> time in a couple of hours. Thus, it is very hard to measure the visible
> performance impact in my machines though the affection of choosing the
> unsuitable migration_target should be negative in theory.
> 
> I feel it's still worth fixing this to at least make the code theoretically
> self-explanatory as it is quite odd an unsuitable migration_target can be
> still migration_target.
> 
> Reported-by: Zhanyuan Hu <huzhanyuan@oppo.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>

LGTM. Thanks.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

> ---
>   v1:
>      move the fix to the specific min_pfn path with respect to Baolin's comment
>   rfc:
>      https://lore.kernel.org/linux-mm/20231129104530.63787-1-v-songbaohua@oppo.com/#t
> 
>   mm/compaction.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 01ba298739dd..de15a2ef0af5 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1611,6 +1611,9 @@ static void fast_isolate_freepages(struct compact_control *cc)
>   						min(pageblock_end_pfn(min_pfn),
>   						    zone_end_pfn(cc->zone)),
>   						cc->zone);
> +					if (page && !suitable_migration_target(cc, page))
> +						page = NULL;
> +
>   					cc->free_pfn = min_pfn;
>   				}
>   			}