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

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Message-ID: <aS13J6U-QMOrwwbs@milan>
Date: Mon, 1 Dec 2025 12:08:23 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: Uladzislau Rezki <urezki@...il.com>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-media@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	John Stultz <jstultz@...gle.com>,
	Maxime Ripard <mripard@...nel.org>
Subject: Re: [PATCH RFC] mm/vmap: map contiguous pages in batches whenever
 possible

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:

>       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.

--
Uladzislau Rezki