[PATCH] mm/vmalloc: request large order pages from buddy allocator

["Vishal Moola (Oracle)" <vishal.moola@gmail.com>]

Sometimes, vm_area_alloc_pages() will want many pages from the buddy
allocator. Rather than making requests to the buddy allocator for at
most 100 pages at a time, we can eagerly request large order pages a
smaller number of times.

We still split the large order pages down to order-0 as the rest of the
vmalloc code (and some callers) depend on it. We still defer to the bulk
allocator and fallback path in case of order-0 pages or failure.

Running 1000 iterations of allocations on a small 4GB system finds:

1000 2mb allocations:
	[Baseline]			[This patch]
	real    46.310s			real    0m34.582
	user    0.001s			user    0.006s
	sys     46.058s			sys     0m34.365s

10000 200kb allocations:
	[Baseline]			[This patch]
	real    56.104s			real    0m43.696
	user    0.001s			user    0.003s
	sys     55.375s			sys     0m42.995s

Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>

-----
RFC:
https://lore.kernel.org/linux-mm/20251014182754.4329-1-vishal.moola@gmail.com/

Changes since rfc:
  - Mask off NO_FAIL in large_gfp
  - Mask off GFP_COMP in large_gfp
There was discussion about warning on and rejecting unsupported GFP
flags in vmalloc, I'll have a separate patch for that.

  - Introduce nr_remaining variable to track total pages
  - Calculate large order as (min(max_order, ilog2())
  - Attempt lower orders on failure before falling back to original path
  - Drop unnecessary fallback comment change
---
 mm/vmalloc.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index adde450ddf5e..0832f944544c 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3619,8 +3619,44 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
 		unsigned int order, unsigned int nr_pages, struct page **pages)
 {
 	unsigned int nr_allocated = 0;
+	unsigned int nr_remaining = nr_pages;
+	unsigned int max_attempt_order = MAX_PAGE_ORDER;
 	struct page *page;
 	int i;
+	gfp_t large_gfp = (gfp &
+		~(__GFP_DIRECT_RECLAIM | __GFP_NOFAIL | __GFP_COMP))
+		| __GFP_NOWARN;
+	unsigned int large_order = ilog2(nr_remaining);
+
+	large_order = min(max_attempt_order, large_order);
+
+	/*
+	 * Initially, attempt to have the page allocator give us large order
+	 * pages. Do not attempt allocating smaller than order chunks since
+	 * __vmap_pages_range() expects physically contigous pages of exactly
+	 * order long chunks.
+	 */
+	while (large_order > order && nr_remaining) {
+		if (nid == NUMA_NO_NODE)
+			page = alloc_pages_noprof(large_gfp, large_order);
+		else
+			page = alloc_pages_node_noprof(nid, large_gfp, large_order);
+
+		if (unlikely(!page)) {
+			max_attempt_order = --large_order;
+			continue;
+		}
+
+		split_page(page, large_order);
+		for (i = 0; i < (1U << large_order); i++)
+			pages[nr_allocated + i] = page + i;
+
+		nr_allocated += 1U << large_order;
+		nr_remaining = nr_pages - nr_allocated;
+
+		large_order = ilog2(nr_remaining);
+		large_order = min(max_attempt_order, large_order);
+	}
 
 	/*
 	 * For order-0 pages we make use of bulk allocator, if
-- 
2.51.0