Messages in this thread |  | | From | Barry Song <> | | Date | Thu, 9 Apr 2026 05:54:55 +0800 | | Subject | Re: [RFC PATCH 5/8] mm/vmalloc: map contiguous pages in batches for vmap() if possible |
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On Wed, Apr 8, 2026 at 10:03 PM Dev Jain <dev.jain@arm.com> wrote: > > > > 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.
If you’re asking about support for non-compound pages, I think that’s fine. My current use case is dma-buf, where pages are compound. I recall discussing this previously with David and Uladzislau.
If you’re working with non-compound pages, I’m happy to add support in the next version. I’m also happy to reuse some of your code and credit you as Co-developed-by if you’re willing. I actually prefer your __vmap_huge() name over my vmap_contig_pages_range().
Does that make sense to you?
> > 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.
I agree, so I had Xueyuan test single pages and highlighted this in the cover letter. There is no regression: "vmap() is 5.6× faster when memory includes some order-8 pages, with no regression observed for order-0 pages."
> > 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.
I don’t see this as a problem at all. Splitting pages does not affect physical or virtual contiguity; it only changes the contents of struct page objects, not the PTE/PMD mappings. For ioremap, there isn’t even a struct page, yet the mappings can still be huge.
Thanks Barry
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