[PATCH v3 10/10] mm: kvmalloc: Add non-blocking support for vmalloc

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

Extend __kvmalloc_node_noprof() to handle non-blocking GFP flags
(GFP_NOWAIT and GFP_ATOMIC). Previously such flags were rejected,
returning NULL. With this change:

- kvmalloc() can fall back to vmalloc() if non-blocking contexts;
- for non-blocking allocations the VM_ALLOW_HUGE_VMAP option is
  disabled, since the huge mapping path still contains might_sleep();
- documentation update to reflect that GFP_NOWAIT and GFP_ATOMIC
  are now supported.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 mm/slub.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 584a5ff1828b..3de0719e24e9 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7018,7 +7018,7 @@ static gfp_t kmalloc_gfp_adjust(gfp_t flags, size_t size)
  * Uses kmalloc to get the memory but if the allocation fails then falls back
  * to the vmalloc allocator. Use kvfree for freeing the memory.
  *
- * GFP_NOWAIT and GFP_ATOMIC are not supported, neither is the __GFP_NORETRY modifier.
+ * GFP_NOWAIT and GFP_ATOMIC are supported, the __GFP_NORETRY modifier is not.
  * __GFP_RETRY_MAYFAIL is supported, and it should be used only if kmalloc is
  * preferable to the vmalloc fallback, due to visible performance drawbacks.
  *
@@ -7027,6 +7027,7 @@ static gfp_t kmalloc_gfp_adjust(gfp_t flags, size_t size)
 void *__kvmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), unsigned long align,
 			     gfp_t flags, int node)
 {
+	bool allow_block;
 	void *ret;
 
 	/*
@@ -7039,16 +7040,22 @@ void *__kvmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), unsigned long align,
 	if (ret || size <= PAGE_SIZE)
 		return ret;
 
-	/* non-sleeping allocations are not supported by vmalloc */
-	if (!gfpflags_allow_blocking(flags))
-		return NULL;
-
 	/* Don't even allow crazy sizes */
 	if (unlikely(size > INT_MAX)) {
 		WARN_ON_ONCE(!(flags & __GFP_NOWARN));
 		return NULL;
 	}
 
+	/*
+	 * For non-blocking the VM_ALLOW_HUGE_VMAP is not used
+	 * because the huge-mapping path in vmalloc contains at
+	 * least one might_sleep() call.
+	 *
+	 * TODO: Revise huge-mapping path to support non-blocking
+	 * flags.
+	 */
+	allow_block = gfpflags_allow_blocking(flags);
+
 	/*
 	 * kvmalloc() can always use VM_ALLOW_HUGE_VMAP,
 	 * since the callers already cannot assume anything
@@ -7056,7 +7063,7 @@ void *__kvmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), unsigned long align,
 	 * protection games.
 	 */
 	return __vmalloc_node_range_noprof(size, align, VMALLOC_START, VMALLOC_END,
-			flags, PAGE_KERNEL, VM_ALLOW_HUGE_VMAP,
+			flags, PAGE_KERNEL, allow_block ? VM_ALLOW_HUGE_VMAP:0,
 			node, __builtin_return_address(0));
 }
 EXPORT_SYMBOL(__kvmalloc_node_noprof);
-- 
2.47.3