Re: [RFC PATCH v3 2/3] mm/migrate: introduce async non-temporal migration mode

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] mm/compaction: enum migrate_mode ordering breaks compact_zone() async check
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commit 047765d971a4b565eac5e2dd50bc528791c2e77e
Author: Huan Nguyen <nhuhuan@google.com>

mm/migrate: introduce async non-temporal migration mode

This commit introduces MIGRATE_ASYNC_NONTEMPORAL to migrate cold folios
using non-temporal stores via folio_mc_copy_nt() in __migrate_folio(), and
adds migrate_is_async() to update callers across compaction, NFS, and
migration.

> diff --git a/include/linux/migrate_mode.h b/include/linux/migrate_mode.h
> index 05102d4d2490e..fd752fddc972e 100644
> --- a/include/linux/migrate_mode.h
> +++ b/include/linux/migrate_mode.h
> @@ -12,6 +12,7 @@ enum migrate_mode {
>  	MIGRATE_ASYNC,
>  	MIGRATE_SYNC_LIGHT,
>  	MIGRATE_SYNC,
> +	MIGRATE_ASYNC_NONTEMPORAL,
>  };

[Severity: Medium]
Does placing MIGRATE_ASYNC_NONTEMPORAL after MIGRATE_SYNC in enum
migrate_mode break the relational check in compact_zone()?

In mm/compaction.c, compact_zone() checks (cc->mode < MIGRATE_SYNC) when a
page fails to migrate within a pageblock:

mm/compaction.c:compact_zone() {
    ...
			/*
			 * If an ASYNC or SYNC_LIGHT fails to migrate a page
			 * within the pageblock_order-aligned block and
			 * fast_find_migrateblock may be used then scan the
			 * remainder of the pageblock.
			 * ...
			 */
			if (!pageblock_aligned(cc->migrate_pfn) &&
			    !cc->ignore_skip_hint && !cc->finish_pageblock &&
			    (cc->mode < MIGRATE_SYNC)) {
				cc->finish_pageblock = true;
    ...
				goto rescan;
			}
    ...
}

Because MIGRATE_ASYNC_NONTEMPORAL has an enum value greater than
MIGRATE_SYNC, (cc->mode < MIGRATE_SYNC) evaluates to false when compact_zone()
runs with cc->mode == MIGRATE_ASYNC_NONTEMPORAL and migration fails
mid-pageblock. That causes compact_zone() to skip setting
cc->finish_pageblock = true and jumping to rescan, leaving the pageblock
partially scanned without marking it skip so fast_find_migrateblock() can
repeatedly revisit it.

Should MIGRATE_ASYNC_NONTEMPORAL be placed before MIGRATE_SYNC_LIGHT in enum
migrate_mode (matching the ordering in include/trace/events/migrate.h), or
should compact_zone() check cc->mode != MIGRATE_SYNC like
isolate_migratepages() does?

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Sashiko AI review · https://sashiko.dev/#/patchset/20261103223025.2482227-1-nhuhuan@google.com?part=2