Re: [PATCH v3 4/6] mm/vmalloc: make set_area_direct_map HUGE_VMAP friendly

On Tue, Sep 08, 2026 at 12:41:48PM +0200, David Hildenbrand (Arm) wrote:

On 9/3/26 11:28, Mike Rapoport (Microsoft) wrote:

quoted

set_area_direct_map() always updates direct map alias permissions in
single page increments.

For HUGE_VMAP areas it's suboptimal. Not only the loop in
set_area_direct_map() needlessly has more iterations (e.g times 512 on
x86), but it also causes fragmentation of the direct map that could be
avoided for the HUGE_VMAP areas populated with large pages.

All pages in an area are always of the same order: either same-order
large pages when VM_ALLOW_HUGE_VMAP is set and all huge pages were
successfully allocated, or order-0 page when VM_ALLOW_HUGE_VMAP is
cleared or when huge pages allocation fails and fallback path is taken.

Instead of updating the direct map permissions for every order-0 page in
an area, use the area's page_order as the loop increment and update the
large pages in one call to set_direct_map_{invalid,default}_noflush().

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 mm/vmalloc.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 5506b180f5c27..6ed6c160abed7 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -3367,12 +3367,15 @@ static inline void set_area_direct_map(const struct vm_struct *area,
 				       int (*set_direct_map)(struct page *page,
 							     unsigned int nr))
 {
-	unsigned long i;
+	unsigned int nr = (1U << vm_area_page_order(area));
+
+	for (unsigned long i = 0; i < area->nr_pages; i += nr) {
+		if (page_address(area->pages[i])) {
+			int err = set_direct_map(area->pages[i], nr);

Do we really expect some pages to have a directmap and others not?

Every page in the area can be allocated separately and
__vmalloc_area_node() has

	if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
		gfp_mask |= __GFP_HIGHMEM;

so generally speaking yes :)

-- 
Cheers,

David

-- 
Sincerely yours,
Mike.