Re: [PATCH RFC] mm/vmap: map contiguous pages in batches whenever possible

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

On Mon, Dec 1, 2025 at 7:08 PM Uladzislau Rezki <urezki@gmail.com> wrote:
>
> On Fri, Nov 28, 2025 at 04:43:54AM +0800, Barry Song wrote:
> > > >
> > > > +     /*
> > > > +      * Some users may allocate pages from high-order down to order 0.
> > > > +      * We roughly check if the first page is a compound page. If so,
> > > > +      * there is a chance to batch multiple pages together.
> > > > +      */
> > > >       if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
> > > > -                     page_shift == PAGE_SHIFT)
> > > > +                     (page_shift == PAGE_SHIFT && !PageCompound(pages[0])))
> > > >
> > > Do we support __GFP_COMP as vmalloc/vmap flag? As i see from latest:
> >
> > This is not the case for vmalloc, but applies to dma-bufs that are allocated
> > using alloc_pages() with GFP_COMP.
> >
> > #define LOW_ORDER_GFP (GFP_HIGHUSER | __GFP_ZERO)
> > #define HIGH_ORDER_GFP  (((GFP_HIGHUSER | __GFP_ZERO | __GFP_NOWARN \
> >                                 | __GFP_NORETRY) & ~__GFP_RECLAIM) \
> >                                 | __GFP_COMP)
> >
> > >
> > > /*
> > >  * See __vmalloc_node_range() for a clear list of supported vmalloc flags.
> > >  * This gfp lists all flags currently passed through vmalloc. Currently,
> > >  * __GFP_ZERO is used by BPF and __GFP_NORETRY is used by percpu. Both drm
> > >  * and BPF also use GFP_USER. Additionally, various users pass
> > >  * GFP_KERNEL_ACCOUNT. Xfs uses __GFP_NOLOCKDEP.
> > >  */
> > > #define GFP_VMALLOC_SUPPORTED (GFP_KERNEL | GFP_ATOMIC | GFP_NOWAIT |\
> > >                                __GFP_NOFAIL |  __GFP_ZERO | __GFP_NORETRY |\
> > >                                GFP_NOFS | GFP_NOIO | GFP_KERNEL_ACCOUNT |\
> > >                                GFP_USER | __GFP_NOLOCKDEP)
> > >
> > > Could you please clarify when PageCompound(pages[0]) returns true?
> > >
> >
> > In this case, dma-buf attempts to allocate as many compound high-order pages
> > as possible, falling back to 0-order allocations if necessary.
> >
> OK, it is folio who uses it.
>
> > Then, dma_buf_vmap() is called by the GPU drivers:
> >
> >  1    404  drivers/accel/amdxdna/amdxdna_gem.c <<amdxdna_gem_obj_vmap>>
> >              dma_buf_vmap(abo->dma_buf, map);
> >    2   1568  drivers/dma-buf/dma-buf.c <<dma_buf_vmap_unlocked>>
> >              ret = dma_buf_vmap(dmabuf, map);
> >    3    354  drivers/gpu/drm/drm_gem_shmem_helper.c
> > <<drm_gem_shmem_vmap_locked>>
> >              ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
> >    4     85  drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> > <<etnaviv_gem_prime_vmap_impl>>
> >              ret = dma_buf_vmap(etnaviv_obj->base.import_attach->dmabuf, &map);
> >    5    433  drivers/gpu/drm/vmwgfx/vmwgfx_blit.c <<map_external>>
> >              ret = dma_buf_vmap(bo->tbo.base.dma_buf, map);
> >    6     88  drivers/gpu/drm/vmwgfx/vmwgfx_gem.c <<vmw_gem_vmap>>
> >              ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
> >
> Thank you for clarification. That would be good to reflect it in the
> commit message. Also, please note that:

Sure.

>
> >       if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
> > -                     page_shift == PAGE_SHIFT)
> > +                     (page_shift == PAGE_SHIFT && !PageCompound(pages[0])))
> >
> we rely on page_shift == PAGE_SHIFT condition for the non-sleep vmalloc()
> allocations(GFP_ATOMIC, GFP_NOWAIT), so we go via vmap_small_pages_range_noflush()
> path. Your patch adds !PageCompound(pages[0]) also. It is not a problem
> since it is vmap() path but we need to comment that.

Sure. Would the following work?

        /*
         * 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 && !PageCompound(pages[0])))
                return vmap_small_pages_range_noflush(addr, end, prot, pages);

Thanks
Barry