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

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Message-ID: <20251223212336.36249-1-21cnbao@gmail.com>
Date: Wed, 24 Dec 2025 10:23:34 +1300
From: Barry Song <21cnbao@...il.com>
To: urezki@...il.com
Cc: 21cnbao@...il.com,
	akpm@...ux-foundation.org,
	david@...nel.org,
	dri-devel@...ts.freedesktop.org,
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Subject: Re: [PATCH] mm/vmalloc: map contiguous pages in batches for vmap() whenever possible

> >  /*
> >   * 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);
> Hm.. If first few pages are order-0 and the rest are compound
> then we do nothing.

Now the dma-buf is allocated in descending order. If page0
is not huge, page1 will not be either. However, I agree that
we may extend support for this case.

>
> >
> > -     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?
> >
> The concern is we mix it with a huge page mapping path. If we want to batch
> v-mapping for page_shift == PAGE_SHIFT case, where "pages" array may contain
> compound pages(folio)(corner case to me), i think we should split it.

I agree this might not be common when the vmap buffer is only
used by the CPU. However, for GPUs, NPUs, and similar devices,
benefiting from larger mappings may be quite common.

Does the code below, which moves batched mapping to vmap(),
address both of your concerns?

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ecbac900c35f..782f2eac8a63 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3501,6 +3501,20 @@ 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;
+
+	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 - map an array of pages into virtually contiguous space
  * @pages: array of page pointers
@@ -3544,10 +3558,21 @@ 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) {
-		vunmap(area->addr);
-		return NULL;
+	for (unsigned int i = 0; i < count; ) {
+		unsigned int shift = PAGE_SHIFT;
+		int err;
+
+		shift += get_vmap_batch_order(pages, count - i, i);
+		err = vmap_range_noflush(addr, addr + (1UL << shift),
+				page_to_phys(pages[i]), pgprot_nx(prot),
+				shift);
+		if (err) {
+			vunmap(area->addr);
+			return NULL;
+		}
+
+		addr += 1UL  << shift;
+		i += 1U << shift;
 	}
 
 	if (flags & VM_MAP_PUT_PAGES) {
-- 
2.48.1

Thanks
Barry