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

[Barry Song <21cnbao@gmail.com>]

On Thu, Dec 18, 2025 at 9:55 PM Uladzislau Rezki <urezki@gmail.com> wrote:
>
> On Thu, Dec 18, 2025 at 02:01:56PM +0100, David Hildenbrand (Red Hat) wrote:
> > On 12/15/25 06:30, Barry Song wrote:
> > > From: Barry Song <v-songbaohua@oppo.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
> > >
> >
> > That's quite a speedup.
> >
> > > Cc: David Hildenbrand <david@kernel.org>
> > > Cc: Uladzislau Rezki <urezki@gmail.com>
> > > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > > Cc: John Stultz <jstultz@google.com>
> > > Cc: Maxime Ripard <mripard@kernel.org>
> > > Tested-by: Tangquan Zheng <zhengtangquan@oppo.com>
> > > Signed-off-by: Barry Song <v-songbaohua@oppo.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;
> >
> > unsigned int, maybe

Right

> >
> > Why are you initializing nr_pages when you overwrite it below?

Right, initializing nr_pages can be dropped.

> >
> > > +
> > > +   /*
> > > +    * Currently, batching is only supported in vmap_pages_range
> > > +    * when page_shift == PAGE_SHIFT.
> >
> > I don't know the code so realizing how we go from page_shift to stride too
> > me a second. Maybe only talk about stride here?
> >
> > OTOH, is "stride" really the right terminology?
> >
> > we calculate it as
> >
> >       stride = 1U << (page_shift - PAGE_SHIFT);
> >
> > page_shift - PAGE_SHIFT should give us an "order". So is this a
> > "granularity" in nr_pages?

This is the case where vmalloc() may realize that it has
high-order pages and therefore calls
vmap_pages_range_noflush() with a page_shift larger than
PAGE_SHIFT. For vmap(), we take a pages array, so
page_shift is always PAGE_SHIFT.

> >
> > Again, I don't know this code, so sorry for the question.
> >
> To me "stride" also sounds unclear.

Thanks, David and Uladzislau. On second thought, this stride may be
redundant, and it should be possible to drop it entirely. This results
in the code below:

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 41dd01e8430c..3962bdcb43e5 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -642,6 +642,20 @@ 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 max_steps, unsigned int idx)
+{
+	unsigned int 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;
+}
+
 /*
  * vmap_pages_range_noflush is similar to vmap_pages_range, but does not
  * flush caches.
@@ -658,20 +672,35 @@ int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
 
 	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)) {
+	for (i = 0; i < nr; ) {
+		unsigned int shift = page_shift;
 		int err;
 
-		err = vmap_range_noflush(addr, addr + (1UL << page_shift),
+		/*
+		 * For vmap() cases, page_shift is always PAGE_SHIFT, even
+		 * if the pages are physically contiguous, they may still
+		 * be mapped in a batch.
+		 */
+		if (page_shift == PAGE_SHIFT)
+			shift += get_vmap_batch_order(pages, nr - i, i);
+		err = vmap_range_noflush(addr, addr + (1UL << shift),
 					page_to_phys(pages[i]), prot,
-					page_shift);
+					shift);
 		if (err)
 			return err;
 
-		addr += 1UL << page_shift;
+		addr += 1UL  << shift;
+		i += 1U << shift;
 	}
 
 	return 0;

Does this look clearer?

Thanks
Barry