On 3/17/21 3:59 PM, Michal Hocko wrote:

On Wed 17-03-21 15:38:35, Oscar Salvador wrote:

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On Wed, Mar 17, 2021 at 03:12:29PM +0100, Michal Hocko wrote:

quoted

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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.

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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? :)