Re: [PATCH v3 21/26] mm/page_alloc: implement FREETYPE_UNMAPPED allocations

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 7/27/26 00:22, Brendan Jackman wrote:
> Currently FREETYPE_UNMAPPED allocs will always fail because, although the
> lists exist to hold them, there is no way to actually create an unmapped
> page block. This commit adds one, and also the logic to map it back
> again when that's needed.
> 
> Doing this at pageblock granularity ensures that the pageblock flags can
> be used to infer which freetype a page belongs to. It also provides nice
> batching of TLB flushes, and also avoids creating too much unnecessary
> TLB fragmentation in the physmap.
> 
> There are some functional requirements for flipping a block:
> 
>  - Unmapping requires a TLB shootdown, meaning IRQs must be enabled.
> 
>  - Updating the pagetables might require allocating a pagetable to break
>    down a huge page. This would deadlock if the zone lock was held.
> 
> This makes allocations that need to change sensitivity _somewhat_
> similar to those that need to fallback to a different migratetype. But,
> the locking requirements mean that this can't just be squashed into the
> existing "fallback" allocator logic, instead a new allocator path just
> for this purpose is needed.
> 
> The new path is assumed to be much cheaper than the really heavyweight
> stuff like compaction and reclaim. But at present it is treated as less
> desirable than the mobility-related "fallback" and "stealing" logic.
> This might turn out to need revision (in particular, maybe it's a
> problem that __rmqueue_steal(), which causes fragmentation, happens
> before __rmqueue_direct_map()), but that should be treated as a subsequent
> optimisation project.
> 
> Adding alloc_flags to gfp_freetype() requires moving it to
> mm/page_alloc.h so it can refer to ALLOC_UNMAPPED. It was already only
> used in internal mm code.
> 
> Now that unmapped pageblocks actually exist, exclude them from
> migration. Migrating unmapped pages via the mermap should be possible
> but that's something to be added later when needed.
> 
> Signed-off-by: Brendan Jackman <jackmanb@google.com>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

A nit below:

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -34,6 +34,7 @@
>  #include <linux/folio_batch.h>
>  #include <linux/memory_hotplug.h>
>  #include <linux/nodemask.h>
> +#include <linux/set_memory.h>
>  #include <linux/vmstat.h>
>  #include <linux/fault-inject.h>
>  #include <linux/compaction.h>
> @@ -1001,6 +1002,26 @@ static void change_pageblock_range(struct page *pageblock_page,
>  	}
>  }
>  
> +/*
> + * Can pages of these two freetypes be combined into a single higher-order free
> + * page?
> + */
> +static inline bool can_merge_freetypes(freetype_t a, freetype_t b)
> +{
> +	if (freetypes_equal(a, b))
> +		return true;
> +
> +	if (!migratetype_is_mergeable(free_to_migratetype(a)) ||
> +	    !migratetype_is_mergeable(free_to_migratetype(b)))
> +		return false;
> +
> +	/*
> +	 * Mustn't "just" merge pages with different freetype flags, changing
> +	 * those requires updating pagetables.
> +	 */
> +	return freetype_flags(a) == freetype_flags(b);
> +}
> +
>  /*
>   * Freeing function for a buddy system allocator.
>   *
> @@ -1069,9 +1090,7 @@ static inline void __free_one_page(struct page *page,
>  			buddy_ft = get_pfnblock_freetype(buddy, buddy_pfn);
>  			buddy_mt = free_to_migratetype(buddy_ft);

There's a comment above this that should probably be moved to
can_merge_freetypes() as well?