Re: [PATCH v1 21/36] mm/cma: refuse handing out non-contiguous page ranges

On 28.08.25 19:28, Lorenzo Stoakes wrote:
> On Thu, Aug 28, 2025 at 12:01:25AM +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.
> 
> I'm guessing this is something that we don't need to worry about in
> reality?

That my theory yes.

> 
>>
>> 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.
>>
>> Reviewed-by: Alexandru Elisei <[email protected]>
>> Signed-off-by: David Hildenbrand <[email protected]>
> 
> LGTM other than refactoring point below.
> 
> CMA stuff looks fine afaict after staring at it for a while, on proviso
> that handing out ranges within the same section is always going to be the
> case.
> 
> Anyway overall,
> 
> LGTM, so:
> 
> Reviewed-by: Lorenzo Stoakes <[email protected]>
> 
> 
>> ---
>>   include/linux/mm.h |  6 ++++++
>>   mm/cma.c           | 39 ++++++++++++++++++++++++---------------
>>   mm/util.c          | 33 +++++++++++++++++++++++++++++++++
>>   3 files changed, 63 insertions(+), 15 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index f6880e3225c5c..2ca1eb2db63ec 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 e56ec64d0567e..813e6dc7b0954 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 +835,24 @@ 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),
>> -					   count, align);
>> -		/* try again with a bit different memory target */
>> -		start = bitmap_no + mask + 1;
>> +		trace_cma_alloc_busy_retry(cma->name, pfn, page, count, align);
>>   	}
>>   out:
>> -	*pagep = page;
>> +	if (!ret)
>> +		*pagep = page;
>>   	return ret;
>>   }
>>
>> @@ -882,7 +891,7 @@ 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);
>>   	}
>>
>>   	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;
> 
> I find this pretty confusing, my test for this is how many times I have to read
> the code to understand what it's doing :)
> 
> So we have something like:
> 
>    (pfn of page)
>     start_pfn        pfn = align UP
>          |                 |
>          v                 v
>   |         section        |
>          <----------------->
>            pfn - start_pfn
> 
> Then check page + (pfn - start_pfn) == pfn_to_page(pfn)
> 
> And loop such that:
> 
>    (pfn of page)
>     start_pfn                                      pfn
>          |                                          |
>          v                                          v
>   |         section        |         section        |
>          <------------------------------------------>
>                          pfn - start_pfn
> 
> Again check page + (pfn - start_pfn) == pfn_to_page(pfn)
> 
> And so on.
> 
> So the logic looks good, but it's just... that took me a hot second to
> parse :)
> 
> I think a few simple fixups
> 
> 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;
> 	/* The PFN of the start of the next section. */
> 	unsigned long pfn = ALIGN(start_pfn, PAGES_PER_SECTION);
> 	/* The page we'd expected to see if the range were contiguous. */
> 	struct page *expected = page + (pfn - start_pfn);
> 
> 	/*
> 	 * The memmap is allocated per memory section. We need to check
> 	 * each involved memory section once.
> 	 */
> 	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION, expected += PAGES_PER_SECTION)
> 		if (unlikely(expected != pfn_to_page(pfn)))
> 			return false;
> 	return true;
> }
> 

Hm, I prefer my variant, especially where the pfn is calculated in the for loop. Likely a
matter of personal taste.

But I can see why skipping the first section might be a surprise when not
having the semantics of ALIGN() in the cache.

So I'll add the following on top:

diff --git a/mm/util.c b/mm/util.c
index 0bf349b19b652..fbdb73aaf35fe 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1303,8 +1303,10 @@ bool page_range_contiguous(const struct page *page, unsigned long nr_pages)
         unsigned long pfn;
  
         /*
-        * The memmap is allocated per memory section. We need to check
-        * each involved memory section once.
+        * The memmap is allocated per memory section, so no need to check
+        * within the first section. However, we need to check each other
+        * spanned memory section once, making sure the first page in a
+        * section could similarly be reached by just iterating pages.
          */
         for (pfn = ALIGN(start_pfn, PAGES_PER_SECTION);
              pfn < end_pfn; pfn += PAGES_PER_SECTION)

Thanks!

-- 
Cheers

David / dhildenb