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

[Yosry Ahmed]

On Tue, Aug 04, 2026 at 11:53:18PM +0000, Yosry Ahmed wrote:
> > @@ -3400,6 +3426,127 @@ static inline void zone_statistics(struct zone *preferred_zone, struct zone *z,
> >  #endif
> >  }
> >  
> > +#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;
> > +
> > +	/*
> > +	 * 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);
> > +	}
> > +	spin_unlock_irqrestore(&zone->lock, irq_flags);
> > +	if (!page)
> > +		return NULL;
> 
> IIUC we only try to change an entire pageblock here, but what if we
> can't? If memory is fragmented enough that many pageblocks have few
> unmapped pages in them, how do we serve a mapped allocation (e.g. a slab
> allocation)?
> 
> We'll go into reclaim/compaction, but there's a chance we'll end up with
> unexpected allocation failures or OOM kills even though we have free
> memory, because unmapped memory is not movable or reclaimable (as of
> now, at least).
> 
> The same could happen if many pageblocks have few mapped but unmovable
> pages in them, and we make an unmapped allocation.
> 
> I wonder if we still need a fallback case where a pageblock contains a
> mix of mapped and unmapped pages. We need to carefully handle such
> pageblocks:
> - For unmapped allocations, we need to unmap the relevant PTEs and
>   potentially do a TLB shootdown (if they were previously mapped). Maybe
>   we should always flush the TLB for simplicity for now.
> - For mapped allocations, we need to map the relevant PTEs. No TLB
>   shootdown should be needed.
> 
> Assuming unmapped allocations are always zeroed by the users on alloc
> and free, we don't need to worry about zeroing pages either way.
> 
> We may want to track the number of unmapped pages in such page blocks to
> now when it's fully mapped or fully unmapped and change its type, but
> maybe this can be a followup if needed.

(I should have probably mentioned that this was surfaced, at least to
me, an internal discussion with Junaid)