[PATCH 2/5] mm/execmem: handle potential allocation errors in the maple tree

["Mike Rapoport (Microsoft)" <rppt@kernel.org>]

execmem_cache_clean() and execmem_cache_alloc_locked() ignore potential
allocation failures in mas_store_gfp().

While in practice they are unlikely to happen, it's better to handle
those errors and ensure the integrity of the ROX cache.

Preallocate the maple tree nodes for the stores that must not fail and
order the maple tree updates so that there won't be any failures once a
tree has been modified.

Assisted-by: copilot:claude-opus-5
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 mm/execmem.c | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/mm/execmem.c b/mm/execmem.c
index ba277790e3132..00dd6324cae01 100644
--- a/mm/execmem.c
+++ b/mm/execmem.c
@@ -149,7 +149,15 @@ static void execmem_cache_clean(struct work_struct *work)
 		if (vm && get_vm_area_size(vm) == size &&
 		    IS_ALIGNED(size, PMD_SIZE) &&
 		    IS_ALIGNED(mas.index, PMD_SIZE)) {
-			mas_store_gfp(&mas, NULL, GFP_KERNEL);
+			/*
+			 * Preallocate to ensure mas_store does not fail
+			 * If there is no memory for the tree update, bail out,
+			 * next execmem_free() might be more lucky
+			 */
+			if (mas_preallocate(&mas, NULL, GFP_KERNEL))
+				break;
+
+			mas_store_prealloc(&mas, NULL);
 			vfree(area);
 		}
 	}
@@ -219,30 +227,34 @@ static void *execmem_cache_alloc_locked(struct execmem_range *range, size_t size
 	addr = mas_free.index;
 	last = mas_free.last;
 
+	mas_set_range(&mas_free, addr, addr + size - 1);
+	if (mas_preallocate(&mas_free, NULL, GFP_KERNEL))
+		return NULL;
+
 	/* insert allocated size to busy_areas at range [addr, addr + size) */
 	mas_set_range(&mas_busy, addr, addr + size - 1);
 	err = mas_store_gfp(&mas_busy, (void *)addr, GFP_KERNEL);
 	if (err)
-		return NULL;
+		goto err_destroy_mas_free;
 
-	mas_store_gfp(&mas_free, NULL, GFP_KERNEL);
+	mas_store_prealloc(&mas_free, NULL);
 	if (area_size > size) {
-		void *ptr = (void *)(addr + size);
-
 		/*
 		 * re-insert remaining free size to free_areas at range
 		 * [addr + size, last]
+		 * the range matches an existing entry, so this cannot allocate
 		 */
+		ptr = (void *)(addr + size);
 		mas_set_range(&mas_free, addr + size, last);
-		err = mas_store_gfp(&mas_free, ptr, GFP_KERNEL);
-		if (err) {
-			mas_store_gfp(&mas_busy, NULL, GFP_KERNEL);
-			return NULL;
-		}
+		mas_store_gfp(&mas_free, ptr, GFP_KERNEL);
 	}
 	ptr = (void *)addr;
 
 	return ptr;
+
+err_destroy_mas_free:
+	mas_destroy(&mas_free);
+	return NULL;
 }
 
 static void *__execmem_cache_alloc(struct execmem_range *range, size_t size)

-- 
2.53.0