Re: [RFC 2/2] mm: page_alloc: per-cpu pageblock buddy allocator

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Apr 03, 2026 at 09:42:08PM -0400, Rik van Riel wrote:
> On Fri, 2026-04-03 at 15:40 -0400, Johannes Weiner wrote:
> > 
> > @@ -755,6 +752,9 @@ struct per_cpu_pages {
> >  #endif
> >   short free_count; /* consecutive free count */
> >  
> > + /* Pageblocks owned by this CPU, for fragment recovery */
> > + struct list_head owned_blocks;
> > +
> >   /* Lists of pages, one per migrate type stored on the pcp-lists */
> >   struct list_head lists[NR_PCP_LISTS];
> >  } ____cacheline_aligned_in_smp;
> > 
> 
> > + /*
> > + * Phase 0: Recover fragments from owned blocks.
> > + *
> > + * The owned_blocks list tracks blocks that have fragments
> > + * sitting in zone buddy (put there by drains). Pull matching
> > + * fragments back to PCP with PagePCPBuddy so they participate
> > + * in merging, instead of claiming fresh blocks and spreading
> > + * fragmentation further.
> > + *
> > + * Only recover blocks matching the requested migratetype.
> > + * After recovery, remove the block from the list -- the drain
> > + * path re-adds it if new fragments arrive.
> > + */
> > + list_for_each_entry_safe(pbd, tmp, &pcp->owned_blocks, cpu_node) {
> > + unsigned long base_pfn, pfn;
> > + int block_mt;
> > +
> > + base_pfn = pbd->block_pfn;
> > + block_mt = pbd_migratetype(pbd);
> > + if (block_mt != migratetype)
> > + continue;
> 
> GIven that you just skip over blocks of the wrong migratetype,
> I wonder if it makes sense to have a different list head for each
> migratetype in the per_cpu_pages struct.

The list is fairly small in practice (pcp->high /
average_free_pages_per_block). I haven't run into any issues with it
so far (although like I said, my tests have been limited).

That said, I also don't think it would be an issue to do that split if
necessary. Additional list heads are 16 bytes per cpu per zone.

Let me keep an eye on this during larger scale tests.