[PATCH v4 10/10] mm: kvmalloc: Add non-blocking support for vmalloc

Uladzislau Rezki (Sony)
2025-10-07T12:21:07Z

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.

Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
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