[PATCH v5 5/6] mm/vmalloc: map contiguous pages in batches for vmap() if possible

[Wen Jiang <jiangwenxiaomi@gmail.com>]

From: "Barry Song (Xiaomi)" <baohua@kernel.org>

In many cases, the pages passed to vmap() may include high-order
pages. 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 physically contiguous pages (regardless of whether
they are compound or non-compound) by scanning with
num_pages_contiguous(), and maps them as a single contiguous block
whenever possible. The mapping order is determined by taking the
minimum of the contiguous page count and the pfn alignment, allowing
graceful degradation when pfn alignment is less than the contiguous
range.

Pages with the same page_shift are coalesced and mapped via
vmap_pages_range_noflush_walk() to avoid page table rewalk.

As users typically allocate memory in descending orders (e.g.
8 → 4 → 0), once an order-0 page is encountered, we stop scanning
for contiguous pages since subsequent pages are likely order-0 as well.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Wen Jiang <jiangwen6@xiaomi.com>
Tested-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
---
 mm/vmalloc.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 85 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d2a4d649af549..db0492151ad08 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3543,6 +3543,89 @@ void vunmap(const void *addr)
 }
 EXPORT_SYMBOL(vunmap);
 
+static inline unsigned int vm_shift(pgprot_t prot, unsigned long size)
+{
+	if (arch_vmap_pmd_supported(prot) && size >= PMD_SIZE)
+		return PMD_SHIFT;
+
+	return arch_vmap_pte_supported_shift(size);
+}
+
+static inline int get_vmap_batch_order(struct page **pages,
+		pgprot_t prot, unsigned int max_steps, unsigned int idx)
+{
+	unsigned int nr_contig;
+	int order;
+
+	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP))
+		return 0;
+
+	nr_contig = num_pages_contiguous(&pages[idx], max_steps);
+	if (nr_contig < 2)
+		return 0;
+
+	order = ilog2(nr_contig);
+
+	/* Limit order by pfn alignment */
+	order = min_t(int, order, __ffs(page_to_pfn(pages[idx])));
+
+	if (vm_shift(prot, PAGE_SIZE << order) == PAGE_SHIFT)
+		return 0;
+
+	return order;
+}
+
+static int vmap_pages_range_batched(unsigned long addr, unsigned long end,
+		pgprot_t prot, struct page **pages)
+{
+	unsigned int count = (end - addr) >> PAGE_SHIFT;
+	unsigned int prev_shift = 0, idx = 0;
+	unsigned long map_addr = addr, batch_end = addr;
+	int err;
+
+	err = kmsan_vmap_pages_range_noflush(addr, end, prot, pages,
+						PAGE_SHIFT, GFP_KERNEL);
+	if (err)
+		goto out;
+
+	for (unsigned int i = 0; i < count; ) {
+		unsigned int shift = PAGE_SHIFT +
+			get_vmap_batch_order(pages, prot, count - i, i);
+
+		if (!i)
+			prev_shift = shift;
+
+		if (shift != prev_shift) {
+			err = vmap_pages_range_noflush_walk(map_addr, batch_end,
+					prot, pages + idx, prev_shift);
+			if (err)
+				goto out;
+			prev_shift = shift;
+			map_addr = batch_end;
+			idx = i;
+		}
+
+		/*
+		 * Once small pages are encountered, the remaining pages
+		 * are likely small as well.
+		 */
+		if (shift == PAGE_SHIFT)
+			break;
+
+		batch_end += 1UL << shift;
+		i += 1U << (shift - PAGE_SHIFT);
+	}
+
+	/* Remaining */
+	if (map_addr < end)
+		err = vmap_pages_range_noflush_walk(map_addr, end,
+				prot, pages + idx, prev_shift);
+
+out:
+	flush_cache_vmap(addr, end);
+	return err;
+}
+
 /**
  * vmap - map an array of pages into virtually contiguous space
  * @pages: array of page pointers
@@ -3586,8 +3669,8 @@ void *vmap(struct page **pages, unsigned int count,
 		return NULL;
 
 	addr = (unsigned long)area->addr;
-	if (vmap_pages_range(addr, addr + size, pgprot_nx(prot),
-				pages, PAGE_SHIFT) < 0) {
+	if (vmap_pages_range_batched(addr, addr + size, pgprot_nx(prot),
+				pages) < 0) {
 		vunmap(area->addr);
 		return NULL;
 	}
-- 
2.34.1