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

[Uladzislau Rezki]

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@xxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> > Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> > Cc: John Stultz <jstultz@xxxxxxxxxx>
> > Cc: Maxime Ripard <mripard@xxxxxxxxxx>
> > Tested-by: Tangquan Zheng <zhengtangquan@xxxxxxxx>
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > ---
> >   * 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@xxxxxxxxx/
> > 
> >   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
> 
> Why are you initializing nr_pages when you overwrite it below?
> 
> > +
> > +	/*
> > +	 * 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?
> 
> Again, I don't know this code, so sorry for the question.
> 
To me "stride" also sounds unclear.

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Uladzislau Rezki