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From:   Rei Yamamoto <yamamoto.rei@jp.fujitsu.com>
To:     linmiaohe@huawei.com
Cc:     akpm@linux-foundation.org, aquini@redhat.com, ddutile@redhat.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        mgorman@techsingularity.net, vvghjk1234@gmail.com,
        yamamoto.rei@jp.fujitsu.com
Subject: Re: [PATCH] mm, compaction: fast_find_migrateblock() should return pfn in the target zone
Date:   Wed, 11 May 2022 16:07:10 +0900
Message-Id: <20220511070710.5576-1-yamamoto.rei@jp.fujitsu.com>
In-Reply-To: <2c2900eb-886a-5bbe-f7c7-9d74a6399893@huawei.com>
References: <2c2900eb-886a-5bbe-f7c7-9d74a6399893@huawei.com>
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On Wed, 11 May 2022 14:25:34 Miaohe Lin wrote:
> On 2022/5/11 12:43, Rei Yamamoto wrote:
>> Prevent returning a pfn outside the target zone in case that not
>> aligned with pageblock boundary.
>> Otherwise isolate_migratepages_block() would handle pages not in
>> the target zone.
>> 
>
> IIUC, the sole caller isolate_migratepages will ensure the pfn won't outside
> the target zone. So the below code change might not be necessary. Or am I miss
> something ?

While block_start_pfn is ensured, this variable is not used as the argument for 
isolate_migratepages_block():
  -----
  static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
  {
  :
          low_pfn = fast_find_migrateblock(cc);
          block_start_pfn = pageblock_start_pfn(low_pfn);
          if (block_start_pfn < cc->zone->zone_start_pfn)
                  block_start_pfn = cc->zone->zone_start_pfn;  <--- block_start_pfn is ensured not outside 
                                                                    the target zone
  :
          block_end_pfn = pageblock_end_pfn(low_pfn);
  :
          for (; block_end_pfn <= cc->free_pfn;
                          fast_find_block = false,
                          cc->migrate_pfn = low_pfn = block_end_pfn,
                          block_start_pfn = block_end_pfn,
                          block_end_pfn += pageblock_nr_pages) {
  :
                  if (isolate_migratepages_block(cc, low_pfn, block_end_pfn,  <--- low_pfn is passed as 
                                                                                   the argument
                                                  isolate_mode))
                          return ISOLATE_ABORT;
  -----

So, the low_pfn passed to isolate_migratepages_block() can be outside the target zone.

Thanks,
Rei