[PATCH v2 22/37] mm/cma: refuse handing out non-contiguous page ranges

David Hildenbrand
2025-09-01T15:56:48Z

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.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/mm.h | 6 ++++++
mm/cma.c | 39 ++++++++++++++++++++++++---------------
mm/util.c | 35 +++++++++++++++++++++++++++++++++++
3 files changed, 65 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..fbdb73aaf35fe 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1280,4 +1280,39 @@ 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, 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)
+ if (unlikely(page + (pfn - start_pfn) != pfn_to_page(pfn)))
+ return false;
+ return true;
+}
+#endif
#endif /* CONFIG_MMU */
--
2.50.1