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.

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>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f6880e3225c5c92448135414083ae8fe2aae6fd1..2ca1eb2db63ecb287d5edc45a547a1c973c2350f 100644 (file)
--- 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 e56ec64d0567eb0e9287cecbc242e816bfdb7be0..813e6dc7b0954864c9ef8cf7adc6a2293241de47 100644 (file)
--- 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 d235b74f7aff78721af5f31fed4b47e912a71a2a..fbdb73aaf35fe87a54909772a409d7774a664a4d 100644 (file)
--- 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 */