linux-kernel - Re: [PATCH v5 2/5] mm,compaction: Let isolate_migratepages

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Date:   Fri, 19 Mar 2021 10:57:47 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/5] mm,compaction: Let
 isolate_migratepages_{range,block} return error codes

On Thu, Mar 18, 2021 at 12:36:52PM +0100, Michal Hocko wrote:
> Yeah, makes sense. I am not a fan of the above form of documentation.
> Btw. maybe renaming the field would be even better, both from the
> intention and review all existing users. I would go with pfn_iter or
> something that wouldn't make it sound like migration specific.

Just to be sure we are on the same page, you meant something like the following
(wrt. comments):

 /*
  * compact_control is used to track pages being migrated and the free pages
  * they are being migrated to during memory compaction. The free_pfn starts
  * at the end of a zone and migrate_pfn begins at the start. Movable pages
  * are moved to the end of a zone during a compaction run and the run
  * completes when free_pfn <= migrate_pfn
  *
  * freepages:           List of free pages to migrate to
  * migratepages:        List of pages that need to be migrated
  * nr_freepages:        Number of isolated free pages
  ...
  */
  struct compact_control {
          struct list_head freepages;
          ...

With the preface that I am not really familiar with compaction code:

About renaming the variable to something else, I wouldn't do it.
I see migrate_pfn being used in contexts where migration gets mentioned,
e.g: 

 /*
  * Briefly search the free lists for a migration source that already has
  * some free pages to reduce the number of pages that need migration
  * before a pageblock is free.
  */
 fast_find_migrateblock(struct compact_control *cc)
 {
  ...
  unsigned long pfn = cc->migrate_pfn;
 }

isolate_migratepages()
 /* Record where migration scanner will be restarted. */

So, I would either stick with it, or add a new 'iter_pfn'/'next_pfn_scan'
field if we feel the need to.

-- 
Oscar Salvador
SUSE L3