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

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Date:   Thu, 18 Mar 2021 10:50:38 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...e.com>, Oscar Salvador <osalvador@...e.de>
Cc:     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 3/17/21 3:59 PM, Michal Hocko wrote:
> On Wed 17-03-21 15:38:35, Oscar Salvador wrote:
>> On Wed, Mar 17, 2021 at 03:12:29PM +0100, Michal Hocko wrote:
>> > > Since isolate_migratepages_block will stop returning the next pfn to be
>> > > scanned, we reuse the cc->migrate_pfn field to keep track of that.
>> > 
>> > This looks hakish and I cannot really tell that users of cc->migrate_pfn
>> > work as intended.

We did check those in detail. Of course it's possible to overlook something...

The alloc_contig_range user never cared about cc->migrate_pfn. compaction
(isolate_migratepages() -> isolate_migratepages_block()) did, and
isolate_migratepages_block() returned the pfn only to be assigned to
cc->migrate_pfn in isolate_migratepages(). I think it's now better that
isolate_migratepages_block() sets it.

>> When discussing this with Vlastimil, I came up with the idea of adding a new
>> field in compact_control struct, e.g: next_pfn_scan to keep track of the next
>> pfn to be scanned.
>> 
>> But Vlastimil made me realize that since cc->migrate_pfn points to that aleady,
>> so we do not need any extra field.

Yes, the first patch had at asome point:

	/* Record where migration scanner will be restarted. */
	cc->migrate_pfn = cc->the_new_field;

Which was a clear sign that the new field is unnecessary.

> This deserves a big fat comment.

Comment where, saying what? :)