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

[Barry Song]

On Wed, Dec 13, 2023 at 9:02 PM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> 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>

Hi Mel,
Thanks!

Hi Andrew,
Given this patch has been in mm-stable,
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?h=mm-stable&id=d19b1a1797
does it still have a chance to collect Mel's tag？

>
> --
> Mel Gorman
> SUSE Labs

Thanks
Barry