[RFC PATCH 5/8] mm/vmalloc: map contiguous pages in batches for vmap() if possible

Barry Song (Xiaomi)
2026-04-08T02:51:52Z

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.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
mm/vmalloc.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 49 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index eba436386929..e8dbfada42bc 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3529,6 +3529,53 @@ void vunmap(const void *addr)
}
EXPORT_SYMBOL(vunmap);

+static inline int get_vmap_batch_order(struct page **pages,
+ unsigned int max_steps, unsigned int idx)
+{
+ unsigned int nr_pages;
+
+ if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP) ||
+ ioremap_max_page_shift == PAGE_SHIFT)
+ return 0;
+
+ nr_pages = compound_nr(pages[idx]);
+ if (nr_pages == 1 || max_steps < nr_pages)
+ return 0;
+
+ if (num_pages_contiguous(&pages[idx], nr_pages) == nr_pages)
+ return compound_order(pages[idx]);
+ return 0;
+}
+
+static int vmap_contig_pages_range(unsigned long addr, unsigned long end,
+ pgprot_t prot, struct page **pages)
+{
+ unsigned int count = (end - addr) >> PAGE_SHIFT;
+ 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, count - i, i);
+
+ err = vmap_range_noflush(addr, addr + (1UL << shift),
+ page_to_phys(pages[i]), prot, shift);
+ if (err)
+ goto out;
+
+ addr += 1UL << shift;
+ i += 1U << (shift - PAGE_SHIFT);
+ }
+
+out:
+ flush_cache_vmap(addr, end);
+ return err;
+}
+
/**
* vmap - map an array of pages into virtually contiguous space
* @pages: array of page pointers
@@ -3572,8 +3619,8 @@ 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_contig_pages_range(addr, addr + size, pgprot_nx(prot),
+ pages) < 0) {
vunmap(area->addr);
return NULL;
}
--
2.39.3 (Apple Git-146)