Subject: Re: [PATCH RFC 21/35] mm/cma: refuse handing out non-contiguous page ranges

From: Alexandru Elisei <[email-masked]>
Date: Tue, 26 Aug 2025 11:45:58 +0100

Hi David,

On Thu, Aug 21, 2025 at 10:06:47PM +0200, David Hildenbrand wrote:
> Let's disallow handing out PFN ranges with non-contiguous pages, so we
> can remove the nth-page usage in __cma_alloc(), and so any callers don't
> have to worry about that either when wanting to blindly iterate pages.
> 
> This is really only a problem in configs with SPARSEMEM but without
> SPARSEMEM_VMEMMAP, and only when we would cross memory sections in some
> cases.
> 
> Will this cause harm? Probably not, because it's mostly 32bit that does
> not support SPARSEMEM_VMEMMAP. If this ever becomes a problem we could
> look into allocating the memmap for the memory sections spanned by a
> single CMA region in one go from memblock.
> 
> Signed-off-by: David Hildenbrand <[email-masked]>
> ---
>  include/linux/mm.h |  6 ++++++
>  mm/cma.c           | 36 +++++++++++++++++++++++-------------
>  mm/util.c          | 33 +++++++++++++++++++++++++++++++++
>  3 files changed, 62 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ef360b72cb05c..f59ad1f9fc792 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -209,9 +209,15 @@ extern unsigned long sysctl_user_reserve_kbytes;
>  extern unsigned long sysctl_admin_reserve_kbytes;
>  
>  #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> +bool page_range_contiguous(const struct page *page, unsigned long nr_pages);
>  #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
>  #else
>  #define nth_page(page,n) ((page) + (n))
> +static inline bool page_range_contiguous(const struct page *page,
> +		unsigned long nr_pages)
> +{
> +	return true;
> +}
>  #endif
>  
>  /* to align the pointer to the (next) page boundary */
> diff --git a/mm/cma.c b/mm/cma.c
> index 2ffa4befb99ab..1119fa2830008 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -780,10 +780,8 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>  				unsigned long count, unsigned int align,
>  				struct page **pagep, gfp_t gfp)
>  {
> -	unsigned long mask, offset;
> -	unsigned long pfn = -1;
> -	unsigned long start = 0;
>  	unsigned long bitmap_maxno, bitmap_no, bitmap_count;
> +	unsigned long start, pfn, mask, offset;
>  	int ret = -EBUSY;
>  	struct page *page = NULL;
>  
> @@ -795,7 +793,7 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>  	if (bitmap_count > bitmap_maxno)
>  		goto out;
>  
> -	for (;;) {
> +	for (start = 0; ; start = bitmap_no + mask + 1) {
>  		spin_lock_irq(&cma->lock);
>  		/*
>  		 * If the request is larger than the available number
> @@ -812,6 +810,22 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>  			spin_unlock_irq(&cma->lock);
>  			break;
>  		}
> +
> +		pfn = cmr->base_pfn + (bitmap_no << cma->order_per_bit);
> +		page = pfn_to_page(pfn);
> +
> +		/*
> +		 * Do not hand out page ranges that are not contiguous, so
> +		 * callers can just iterate the pages without having to worry
> +		 * about these corner cases.
> +		 */
> +		if (!page_range_contiguous(page, count)) {
> +			spin_unlock_irq(&cma->lock);
> +			pr_warn_ratelimited("%s: %s: skipping incompatible area [0x%lx-0x%lx]",
> +					    __func__, cma->name, pfn, pfn + count - 1);
> +			continue;
> +		}
> +
>  		bitmap_set(cmr->bitmap, bitmap_no, bitmap_count);
>  		cma->available_count -= count;
>  		/*
> @@ -821,29 +835,25 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>  		 */
>  		spin_unlock_irq(&cma->lock);
>  
> -		pfn = cmr->base_pfn + (bitmap_no << cma->order_per_bit);
>  		mutex_lock(&cma->alloc_mutex);
>  		ret = alloc_contig_range(pfn, pfn + count, ACR_FLAGS_CMA, gfp);
>  		mutex_unlock(&cma->alloc_mutex);
> -		if (ret == 0) {
> -			page = pfn_to_page(pfn);
> +		if (!ret)
>  			break;
> -		}
>  
>  		cma_clear_bitmap(cma, cmr, pfn, count);
>  		if (ret != -EBUSY)
>  			break;
>  
>  		pr_debug("%s(): memory range at pfn 0x%lx %p is busy, retrying\n",
> -			 __func__, pfn, pfn_to_page(pfn));
> +			 __func__, pfn, page);
>  
>  		trace_cma_alloc_busy_retry(cma->name, pfn, pfn_to_page(pfn),

Nitpick: I think you already have the page here.

>  					   count, align);
> -		/* try again with a bit different memory target */
> -		start = bitmap_no + mask + 1;
>  	}
>  out:
> -	*pagep = page;
> +	if (!ret)
> +		*pagep = page;
>  	return ret;
>  }
>  
> @@ -882,7 +892,7 @@ static struct page *__cma_alloc(struct cma *cma, unsigned long count,
>  	 */
>  	if (page) {
>  		for (i = 0; i < count; i++)
> -			page_kasan_tag_reset(nth_page(page, i));
> +			page_kasan_tag_reset(page + i);

Had a look at it, not very familiar with CMA, but the changes look equivalent to
what was before. Not sure that's worth a Reviewed-by tag, but here it in case
you want to add it:

Reviewed-by: Alexandru Elisei <[email-masked]>

Just so I can better understand the problem being fixed, I guess you can have
two consecutive pfns with non-consecutive associated struct page if you have two
adjacent memory sections spanning the same physical memory region, is that
correct?

Thanks,
Alex

>  	}
>  
>  	if (ret && !(gfp & __GFP_NOWARN)) {
> diff --git a/mm/util.c b/mm/util.c
> index d235b74f7aff7..0bf349b19b652 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1280,4 +1280,37 @@ unsigned int folio_pte_batch(struct folio *folio, pte_t *ptep, pte_t pte,
>  {
>  	return folio_pte_batch_flags(folio, NULL, ptep, &pte, max_nr, 0);
>  }
> +
> +#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> +/**
> + * page_range_contiguous - test whether the page range is contiguous
> + * @page: the start of the page range.
> + * @nr_pages: the number of pages in the range.
> + *
> + * Test whether the page range is contiguous, such that they can be iterated
> + * naively, corresponding to iterating a contiguous PFN range.
> + *
> + * This function should primarily only be used for debug checks, or when
> + * working with page ranges that are not naturally contiguous (e.g., pages
> + * within a folio are).
> + *
> + * Returns true if contiguous, otherwise false.
> + */
> +bool page_range_contiguous(const struct page *page, unsigned long nr_pages)
> +{
> +	const unsigned long start_pfn = page_to_pfn(page);
> +	const unsigned long end_pfn = start_pfn + nr_pages;
> +	unsigned long pfn;
> +
> +	/*
> +	 * The memmap is allocated per memory section. We need to check
> +	 * each involved memory section once.
> +	 */
> +	for (pfn = ALIGN(start_pfn, PAGES_PER_SECTION);
> +	     pfn < end_pfn; pfn += PAGES_PER_SECTION)
> +		if (unlikely(page + (pfn - start_pfn) != pfn_to_page(pfn)))
> +			return false;
> +	return true;
> +}
> +#endif
>  #endif /* CONFIG_MMU */
> -- 
> 2.50.1
> 
>