linux-kernel - [PATCH] mm/vmalloc: map contiguous pages in batches for vmap() whenever possible

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Message-Id: <20251215053050.11599-1-21cnbao@gmail.com>
Date: Mon, 15 Dec 2025 13:30:50 +0800
From: Barry Song <21cnbao@...il.com>
To: akpm@...ux-foundation.org,
	linux-mm@...ck.org
Cc: dri-devel@...ts.freedesktop.org,
	jstultz@...gle.com,
	linaro-mm-sig@...ts.linaro.org,
	linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org,
	Barry Song <v-songbaohua@...o.com>,
	David Hildenbrand <david@...nel.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Maxime Ripard <mripard@...nel.org>,
	Tangquan Zheng <zhengtangquan@...o.com>
Subject: [PATCH] mm/vmalloc: map contiguous pages in batches for vmap() whenever possible

From: Barry Song <v-songbaohua@...o.com>

In many cases, the pages passed to vmap() may include high-order
pages allocated with __GFP_COMP flags. For example, the systemheap
often allocates pages in descending order: order 8, then 4, then 0.
Currently, vmap() iterates over every page individually—even pages
inside a high-order block are handled one by one.

This patch detects high-order pages and maps them as a single
contiguous block whenever possible.

An alternative would be to implement a new API, vmap_sg(), but that
change seems to be large in scope.

When vmapping a 128MB dma-buf using the systemheap, this patch
makes system_heap_do_vmap() roughly 17× faster.

W/ patch:
[   10.404769] system_heap_do_vmap took 2494000 ns
[   12.525921] system_heap_do_vmap took 2467008 ns
[   14.517348] system_heap_do_vmap took 2471008 ns
[   16.593406] system_heap_do_vmap took 2444000 ns
[   19.501341] system_heap_do_vmap took 2489008 ns

W/o patch:
[    7.413756] system_heap_do_vmap took 42626000 ns
[    9.425610] system_heap_do_vmap took 42500992 ns
[   11.810898] system_heap_do_vmap took 42215008 ns
[   14.336790] system_heap_do_vmap took 42134992 ns
[   16.373890] system_heap_do_vmap took 42750000 ns

Cc: David Hildenbrand <david@...nel.org>
Cc: Uladzislau Rezki <urezki@...il.com>
Cc: Sumit Semwal <sumit.semwal@...aro.org>
Cc: John Stultz <jstultz@...gle.com>
Cc: Maxime Ripard <mripard@...nel.org>
Tested-by: Tangquan Zheng <zhengtangquan@...o.com>
Signed-off-by: Barry Song <v-songbaohua@...o.com>
---
 * diff with rfc:
 Many code refinements based on David's suggestions, thanks!
 Refine comment and changelog according to Uladzislau, thanks!
 rfc link:
 https://lore.kernel.org/linux-mm/20251122090343.81243-1-21cnbao@gmail.com/

 mm/vmalloc.c | 45 +++++++++++++++++++++++++++++++++++++++------
 1 file changed, 39 insertions(+), 6 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 41dd01e8430c..8d577767a9e5 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -642,6 +642,29 @@ static int vmap_small_pages_range_noflush(unsigned long addr, unsigned long end,
 	return err;
 }
 
+static inline int get_vmap_batch_order(struct page **pages,
+		unsigned int stride, unsigned int max_steps, unsigned int idx)
+{
+	int nr_pages = 1;
+
+	/*
+	 * Currently, batching is only supported in vmap_pages_range
+	 * when page_shift == PAGE_SHIFT.
+	 */
+	if (stride != 1)
+		return 0;
+
+	nr_pages = compound_nr(pages[idx]);
+	if (nr_pages == 1)
+		return 0;
+	if (max_steps < nr_pages)
+		return 0;
+
+	if (num_pages_contiguous(&pages[idx], nr_pages) == nr_pages)
+		return compound_order(pages[idx]);
+	return 0;
+}
+
 /*
  * vmap_pages_range_noflush is similar to vmap_pages_range, but does not
  * flush caches.
@@ -655,23 +678,33 @@ int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
 		pgprot_t prot, struct page **pages, unsigned int page_shift)
 {
 	unsigned int i, nr = (end - addr) >> PAGE_SHIFT;
+	unsigned int stride;
 
 	WARN_ON(page_shift < PAGE_SHIFT);
 
+	/*
+	 * For vmap(), users may allocate pages from high orders down to
+	 * order 0, while always using PAGE_SHIFT as the page_shift.
+	 * We first check whether the initial page is a compound page. If so,
+	 * there may be an opportunity to batch multiple pages together.
+	 */
 	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
-			page_shift == PAGE_SHIFT)
+			(page_shift == PAGE_SHIFT && !PageCompound(pages[0])))
 		return vmap_small_pages_range_noflush(addr, end, prot, pages);
 
-	for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
-		int err;
+	stride = 1U << (page_shift - PAGE_SHIFT);
+	for (i = 0; i < nr; ) {
+		int err, order;
 
-		err = vmap_range_noflush(addr, addr + (1UL << page_shift),
+		order = get_vmap_batch_order(pages, stride, nr - i, i);
+		err = vmap_range_noflush(addr, addr + (1UL << (page_shift + order)),
 					page_to_phys(pages[i]), prot,
-					page_shift);
+					page_shift + order);
 		if (err)
 			return err;
 
-		addr += 1UL << page_shift;
+		addr += 1UL  << (page_shift + order);
+		i += 1U << (order + page_shift - PAGE_SHIFT);
 	}
 
 	return 0;
-- 
2.39.3 (Apple Git-146)