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

[Mel Gorman]

On Thu, Dec 07, 2023 at 12:00:54AM +1300, 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@xxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

-- 
Mel Gorman
SUSE Labs