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

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Date:   Thu, 18 Mar 2021 11:22:44 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Oscar Salvador <osalvador@...e.de>,
        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 18-03-21 10:50:38, Vlastimil Babka wrote:
> 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? :)

E.g. something like the following
diff --git a/mm/internal.h b/mm/internal.h
index 1432feec62df..6c5a9066adf0 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -225,7 +225,13 @@ struct compact_control {
 	unsigned int nr_freepages;	/* Number of isolated free pages */
 	unsigned int nr_migratepages;	/* Number of pages to migrate */
 	unsigned long free_pfn;		/* isolate_freepages search base */
-	unsigned long migrate_pfn;	/* isolate_migratepages search base */
+	unsigned long migrate_pfn;	/* Acts as an in/out parameter to page
+					 * isolation.
+					 * isolate_migratepages uses it as a search base.
+					 * isolate_migratepages_block will update the
+					 * value the next pfn after the last isolated
+					 * one.
+					 */
 	unsigned long fast_start_pfn;	/* a pfn to start linear scan from */
 	struct zone *zone;
 	unsigned long total_migrate_scanned;

Btw isolate_migratepages_block still has this comment which needs
updating
"The cc->migrate_pfn field is neither read nor updated."
-- 
Michal Hocko
SUSE Labs