Re: [RFC PATCH 3/5] mm/page_alloc: convert any movable pageblock on a non-movable allocation

[Rik van Riel <riel@surriel.com>]

On Wed, 2026-10-14 at 22:48 +0200, Johannes Weiner wrote:
> On Tue, Oct 06, 2026 at 10:12:48PM -0400, Rik van Riel wrote:
> > try_to_claim_block() converts a movable pageblock for a non-movable
> > allocation only when half its pages are free or compatible.
> > Otherwise the steal path takes pages without converting, and
> > non-movable content sits in a block still typed movable.
> > 
> > Convert a movable block on any non-movable claim, however few of
> > its pages are free, so the block's type matches the allocation it
> > serves.
> > 
> > When other pages in the block are freed later, they end up on the
> > non-movable free lists, directing more non-movable allocations to
> > the already non-movable page blocks.
> > 
> > Converting a non-movable pageblock to movable is made stricter:
> > a movable claim takes a non-movable block only when every page
> > is free or movable.
> > 
> > Blocks straddling a zone edge are never claimed;
> > zone_spans_pageblock() tests for a block wholly inside its zone.
> > 
> > Assisted-by: LLM
> > Signed-off-by: Rik van Riel <riel@surriel.com>
> 
> I like this in theory, but wonder how this behaves in practice.
> 
> The block is claimed, but the movable constituents aren't proactively
> kicked out.

I have code for that, too, but it seems like
it's a lot easier to get things reviewed in
small portions.

If you want I can resend this as a larger
series, with:
1) the pageblock claiming
2) per migrate type free lists
3) evacuation of movable pages from unmovable 
   pageblocks, when the kernel free lists fall 
   below a watermark

That would be the full extent of the 2MB granularity
fragmentation avoidance stuff I have here.

Just let me know your preference.

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