Re: [PATCH 4/5] mm: workingset: move the stats flush into workingset_test_recent()

[Yosry Ahmed]

On Thu, Sep 21, 2023 at 4:06 AM kernel test robot <lkp@xxxxxxxxx> wrote:
>
> Hi Yosry,
>
> kernel test robot noticed the following build errors:
>
> [auto build test ERROR on akpm-mm/mm-everything]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Yosry-Ahmed/mm-memcg-change-flush_next_time-to-flush_last_time/20230921-161246
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> patch link:    https://lore.kernel.org/r/20230921081057.3440885-5-yosryahmed%40google.com
> patch subject: [PATCH 4/5] mm: workingset: move the stats flush into workingset_test_recent()
> config: powerpc-allnoconfig (https://download.01.org/0day-ci/archive/20230921/202309211829.Efuqg8NE-lkp@xxxxxxxxx/config)
> compiler: powerpc-linux-gcc (GCC) 13.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20230921/202309211829.Efuqg8NE-lkp@xxxxxxxxx/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202309211829.Efuqg8NE-lkp@xxxxxxxxx/
>
> All errors (new ones prefixed by >>):
>
>    mm/workingset.c: In function 'workingset_test_recent':
> >> mm/workingset.c:461:32: error: invalid use of undefined type 'struct mem_cgroup'
>      461 |         css_get(&eviction_memcg->css);
>          |                                ^~
>

Ah yes, I cannot dereference the memcg pointer here. I think we want
mem_cgroup_get_from_id (a getter version of mem_cgroup_from_id) or
mem_cgroup_get (similar to mem_cgroup_put) to have stubs when
!CONFIG_MEMCG. I will do this change in the next version, but I'll
wait for feedback on this version first.

>
> vim +461 mm/workingset.c
>
>    405
>    406  /**
>    407   * workingset_test_recent - tests if the shadow entry is for a folio that was
>    408   * recently evicted. Also fills in @workingset with the value unpacked from
>    409   * shadow.
>    410   * @shadow: the shadow entry to be tested.
>    411   * @file: whether the corresponding folio is from the file lru.
>    412   * @workingset: where the workingset value unpacked from shadow should
>    413   * be stored.
>    414   *
>    415   * Return: true if the shadow is for a recently evicted folio; false otherwise.
>    416   */
>    417  bool workingset_test_recent(void *shadow, bool file, bool *workingset)
>    418  {
>    419          struct mem_cgroup *eviction_memcg;
>    420          struct lruvec *eviction_lruvec;
>    421          unsigned long refault_distance;
>    422          unsigned long workingset_size;
>    423          unsigned long refault;
>    424          int memcgid;
>    425          struct pglist_data *pgdat;
>    426          unsigned long eviction;
>    427
>    428          rcu_read_lock();
>    429
>    430          if (lru_gen_enabled()) {
>    431                  bool recent = lru_gen_test_recent(shadow, file,
>    432                                                    &eviction_lruvec, &eviction,
>    433                                                    workingset);
>    434                  rcu_read_unlock();
>    435                  return recent;
>    436          }
>    437
>    438          unpack_shadow(shadow, &memcgid, &pgdat, &eviction, workingset);
>    439          eviction <<= bucket_order;
>    440
>    441          /*
>    442           * Look up the memcg associated with the stored ID. It might
>    443           * have been deleted since the folio's eviction.
>    444           *
>    445           * Note that in rare events the ID could have been recycled
>    446           * for a new cgroup that refaults a shared folio. This is
>    447           * impossible to tell from the available data. However, this
>    448           * should be a rare and limited disturbance, and activations
>    449           * are always speculative anyway. Ultimately, it's the aging
>    450           * algorithm's job to shake out the minimum access frequency
>    451           * for the active cache.
>    452           *
>    453           * XXX: On !CONFIG_MEMCG, this will always return NULL; it
>    454           * would be better if the root_mem_cgroup existed in all
>    455           * configurations instead.
>    456           */
>    457          eviction_memcg = mem_cgroup_from_id(memcgid);
>    458          if (!mem_cgroup_disabled() && !eviction_memcg)
>    459                  return false;
>    460
>  > 461          css_get(&eviction_memcg->css);
>    462          rcu_read_unlock();
>    463
>    464          /* Flush stats (and potentially sleep) outside the RCU read section */
>    465          mem_cgroup_flush_stats_ratelimited();
>    466
>    467          eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
>    468          refault = atomic_long_read(&eviction_lruvec->nonresident_age);
>    469
>    470          /*
>    471           * Calculate the refault distance
>    472           *
>    473           * The unsigned subtraction here gives an accurate distance
>    474           * across nonresident_age overflows in most cases. There is a
>    475           * special case: usually, shadow entries have a short lifetime
>    476           * and are either refaulted or reclaimed along with the inode
>    477           * before they get too old.  But it is not impossible for the
>    478           * nonresident_age to lap a shadow entry in the field, which
>    479           * can then result in a false small refault distance, leading
>    480           * to a false activation should this old entry actually
>    481           * refault again.  However, earlier kernels used to deactivate
>    482           * unconditionally with *every* reclaim invocation for the
>    483           * longest time, so the occasional inappropriate activation
>    484           * leading to pressure on the active list is not a problem.
>    485           */
>    486          refault_distance = (refault - eviction) & EVICTION_MASK;
>    487
>    488          /*
>    489           * Compare the distance to the existing workingset size. We
>    490           * don't activate pages that couldn't stay resident even if
>    491           * all the memory was available to the workingset. Whether
>    492           * workingset competition needs to consider anon or not depends
>    493           * on having free swap space.
>    494           */
>    495          workingset_size = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE);
>    496          if (!file) {
>    497                  workingset_size += lruvec_page_state(eviction_lruvec,
>    498                                                       NR_INACTIVE_FILE);
>    499          }
>    500          if (mem_cgroup_get_nr_swap_pages(eviction_memcg) > 0) {
>    501                  workingset_size += lruvec_page_state(eviction_lruvec,
>    502                                                       NR_ACTIVE_ANON);
>    503                  if (file) {
>    504                          workingset_size += lruvec_page_state(eviction_lruvec,
>    505                                                       NR_INACTIVE_ANON);
>    506                  }
>    507          }
>    508
>    509          mem_cgroup_put(eviction_memcg);
>    510          return refault_distance <= workingset_size;
>    511  }
>    512
>
> --
> 0-DAY CI Kernel Test Service
> https://github.com/intel/lkp-tests/wiki