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
