Re: [PATCH v2 1/3] mm: page_alloc: add a watermark check that knows the migratetype

[Kiryl Shutsemau <kas@kernel.org>]

On Thu, Nov 05, 2026 at 05:06:39PM +0000, Harry Yoo wrote:
> On Mon, Nov 02, 2026 at 09:39:34AM +0000, Kiryl Shutsemau (Meta) wrote:
> > The high-order check in __zone_watermark_ok() accepts a free page of any
> > migratetype.  Under ALLOC_NOFRAGMENT, a request below pageblock_order can
> > only take a page of its own migratetype or claim a whole free block, so
> > the check can pass when the allocation cannot succeed.
> > 
> > Telling those apart needs the request's migratetype, but not every check
> > wants them told apart.  A check whether an allocation can be served now
> > has to count only the pages it can take.
> >
> > A check whether there is enough free memory counts every migratetype:
> > kswapd and kcompactd balancing a zone, order-0 thresholds, the
> > slowpath's forecast whether reclaim and compaction can still make
> > progress.  Reclaim and compaction work across migratetypes to free whole
> > pageblocks.
> 
> nit: perhaps we could make the changelog above bit more concise.
> 
> I think the necessary bit here is "With defrag_mode, allocations cannot
> steal a page from pageblocks of other migrate type, thus
> __zone_watermark_ok() needs to take that into account."


It is ALLOC_NOFRAGMENT, not defrag_mode: Normal zone allocations get it
on the first attempt when DMA32 is populated, with defrag_mode off. And
such an allocation can still claim a whole free pageblock of another
migratetype.

But fair, it can be shorter. A tighter version is below; it also says
why kcompactd passes MIGRATE_MOVABLE.

> nit: maybe let's set cc.migratetype and pass it?

The value is not read there: __zone_watermark_ok_mt() uses the
migratetype only under ALLOC_NOFRAGMENT, and kcompactd never passes it.
kcompactd_node_suitable() makes the same call and has no compact_control
to take it from.

Thanks for the review.

The new changelog:

	The high-order check in __zone_watermark_ok() accepts a free page of any
	migratetype.  Under ALLOC_NOFRAGMENT, a request below pageblock_order can
	only take a page of its own migratetype or claim a whole free block, so
	the check can pass when the allocation cannot succeed.

	Only a check whether one allocation can be served now should count just
	the pages it can take.  A check whether a zone has enough free memory
	serves no single allocation and counts every migratetype: reclaim and
	compaction free whole pageblocks across migratetypes.

	Add __zone_watermark_ok_mt(), which takes the migratetype, for the first
	kind:

	  - the allocator fast path, zone_watermark_fast();
	  - its recheck after node reclaim in get_page_from_freelist();
	  - compaction's up-front check, compaction_suit_allocation_order().

	__zone_watermark_ok() wraps it for the second kind.  kcompactd shares
	compaction_suit_allocation_order() with direct compaction and passes
	MIGRATE_MOVABLE, which is not read without ALLOC_NOFRAGMENT.

	Declare both in mm/internal.h: nothing outside mm uses them.

	Nothing reads the migratetype yet: no functional change.  Preparation for
	making the high-order check honour ALLOC_NOFRAGMENT.

Looks good?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov