mm/cma: refuse handing out non-contiguous page ranges

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.

[david@redhat.com: we can have NUMMU configs with SPARSEMEM enabled]
  Link: https://lkml.kernel.org/r/6ec933b1-b3f7-41c0-95d8-e518bb87375e@redhat.com
Link: https://lkml.kernel.org/r/20250901150359.867252-23-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 24 insertions, 15 deletions

diff --git a/mm/cma.c b/mm/cma.c
index e56ec64d0567..813e6dc7b095 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)) {