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

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Message-ID: <CAGsJ_4z21fN2KSg6jt_qveYgrKZw1Vsi_kb+uhed=RJAbujMHw@mail.gmail.com>
Date: Fri, 28 Nov 2025 04:43:54 +0800
From: Barry Song <21cnbao@...il.com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: 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

> >
> > +     /*
> > +      * 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.

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);

Thanks
Barry