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

[Dev Jain <dev.jain@arm.com>]

On 08/04/26 8:21 am, Barry Song (Xiaomi) wrote:
> 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;

This assumes that the page array passed to vmap() will have compound pages
if it is a higher order allocation.

See rb_alloc_aux_page(). It gets higher-order allocations without passing
GFP_COMP.

That is why my implementation does not assume anything about the property
of the pages.

Also it may be useful to do regression-testing for the common case of
vmap() with a single page (assuming it is common, I don't know), in
which case we may have to special case it.

My implementation requires opting in with VM_ALLOW_HUGE_VMAP - I suspect
you may run into problems if you make vmap() do huge-mappings as best-effort
by default. I am guessing this because ...

Drivers can operate on individual pages, so vmalloc() calls split_page()
and then does the block/cont mappings. This same issue should be present
with vmap() too? In which case if we are to do huge-mappings by default
then we can do split_page() after detecting contiguous chunks.

But ... that may create problems for the caller of vmap() - vmap now
has the changed the properties of the pages.


> +
> +	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;
>  	}