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

[Yosry Ahmed]

On Sun, Jul 26, 2026 at 10:22:54PM +0000, 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@xxxxxxxxxx>
> ---
>  mm/Kconfig      |   7 +-
>  mm/compaction.c |   8 ++-
>  mm/page_alloc.c | 214 +++++++++++++++++++++++++++++++++++++++++++++++++-------
>  mm/page_alloc.h |  13 +++-
>  mm/page_owner.c |   6 +-
>  5 files changed, 215 insertions(+), 33 deletions(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index aa5f041ce1a67..6a89cbf8e8c6e 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1527,9 +1527,10 @@ config MERMAP_KUNIT_TEST
>  
>  	  If unsure, say N.
>  
> +config PAGE_ALLOC_UNMAPPED
> +	bool
> +	depends on !HIGHMEM
> +
>  source "mm/damon/Kconfig"
>  
>  endmenu
> -
> -config PAGE_ALLOC_UNMAPPED
> -	bool
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 67b01af024e17..c9eb3947ffc79 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1393,6 +1393,9 @@ static bool suitable_migration_source(struct compact_control *cc,

There is a direct compaction check above the context lines:

	/*
         * Background compaction produces blocks for the zone at
         * large, with no particular allocation context. Allow all
         * block types, including CMA.
         */
        if (!cc->direct_compaction)
                return true;

>  	block_ft = get_pageblock_freetype(page);
>  	block_mt = free_to_migratetype(block_ft);
>  
> +	if (freetype_unmapped(get_pageblock_freetype(page)))
> +		return false;

..and I think this freetype_unmapped() check should be moved above it.
Otherwise kcompactd (for e.g.) won't check for unmapped pages and
crashes.

[..]

> +#ifdef CONFIG_PAGE_ALLOC_UNMAPPED
> +/* Try to allocate a page by mapping/unmapping a block from the direct map. */
> +static inline struct page *
> +__rmqueue_direct_map(struct zone *zone, unsigned int request_order,
> +		     unsigned int alloc_flags, freetype_t freetype)
> +{
> +	unsigned int ft_flags_other = freetype_flags(freetype) ^ FREETYPE_UNMAPPED;
> +	freetype_t ft_other = migrate_to_freetype(free_to_migratetype(freetype),
> +						  ft_flags_other);
> +	bool want_mapped = !(freetype_flags(freetype) & FREETYPE_UNMAPPED);
> +	enum rmqueue_mode rmqm = RMQUEUE_NORMAL;
> +	unsigned long irq_flags;
> +	int nr_pageblocks, nr_freed;
> +	struct page *page;
> +	int alloc_order;
> +	int err;
> +
> +	if (freetype_idx(ft_other) < 0)
> +		return NULL;

For a movable mapped allocation, ft_other will end up being movable
unmapped, which will immediately fail this check before trying to
convert a pageblock. This leads to premature OOM kills.

> +
> +	/*
> +	 * Might need a TLB shootdown. Even if IRQs are on this isn't
> +	 * safe if the caller holds a lock (in case the other CPUs need that
> +	 * lock to handle the shootdown IPI).
> +	 */
> +	if (alloc_flags & ALLOC_NOBLOCK)
> +		return NULL;
> +
> +	if (!can_set_direct_map() || alloc_flags & ALLOC_NOLOCK)
> +		return NULL;
> +
> +	lockdep_assert(!irqs_disabled() || unlikely(early_boot_irqs_disabled));
> +
> +	/*
> +	 * Need to [un]map a whole pageblock (otherwise it might require
> +	 * allocating pagetables). First allocate it.
> +	 */
> +	alloc_order = max(request_order, pageblock_order);
> +	nr_pageblocks = 1 << (alloc_order - pageblock_order);
> +	spin_lock_irqsave(&zone->lock, irq_flags);
> +	/* First try a block that already has the right migratetype. */
> +	page = __rmqueue(zone, alloc_order, ft_other, alloc_flags, &rmqm);
> +	if (!page) {
> +		/* Fallback to changing a block's migratetype. */
> +		rmqm = RMQUEUE_CLAIM;
> +		page = __rmqueue(zone, alloc_order, ft_other, alloc_flags, &rmqm);

IIUC, the first call to __rmqueue() will already try RMQUEUE_CLAIM. Is
this redundant?