Re: [PATCH v8 01/23] dma-direct: return struct page from dma_direct_alloc_from_pool()

[Leon Romanovsky]

On Tue, Jul 21, 2026 at 07:50:10PM +0530, Aneesh Kumar K.V wrote:
> Leon Romanovsky <leon@xxxxxxxxxx> writes:
> 
> > On Fri, Jul 17, 2026 at 11:34:19PM +0530, Aneesh Kumar K.V (Arm) wrote:
> >> Commit 5b138c534fda ("dma-direct: factor out a dma_direct_alloc_from_pool
> >> helper") changed dma_direct_alloc_from_pool() to return the CPU address
> >> from dma_alloc_from_pool(). That fits dma_direct_alloc(), but
> >> dma_direct_alloc_pages() also uses the helper and expects a struct page *.
> >> 
> >> Fix this by making dma_direct_alloc_from_pool() return the struct page *
> >> again, and pass the CPU address back through an out-parameter for the
> >> dma_direct_alloc() caller.
> >> 
> >> Fixes: 5b138c534fda ("dma-direct: factor out a dma_direct_alloc_from_pool helper")
> >> Cc: stable@xxxxxxxxxxxxxxx
> >> Tested-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> >> Tested-by: Mostafa Saleh <smostafa@xxxxxxxxxx>
> >> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> >> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@xxxxxxxxxx>
> >> ---
> >>  kernel/dma/direct.c | 18 ++++++++++--------
> >>  1 file changed, 10 insertions(+), 8 deletions(-)
> >> 
> >> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> >> index d8219efe3273..363d984d90e7 100644
> >> --- a/kernel/dma/direct.c
> >> +++ b/kernel/dma/direct.c
> >> @@ -164,22 +164,21 @@ static bool dma_direct_use_pool(struct device *dev, gfp_t gfp)
> >>  	return !gfpflags_allow_blocking(gfp) && !is_swiotlb_for_alloc(dev);
> >>  }
> >>  
> >> -static void *dma_direct_alloc_from_pool(struct device *dev, size_t size,
> >> -		dma_addr_t *dma_handle, gfp_t gfp)
> >> +static struct page *dma_direct_alloc_from_pool(struct device *dev, size_t size,
> >> +		dma_addr_t *dma_handle, void **cpu_addr, gfp_t gfp)
> >>  {
> >>  	struct page *page;
> >>  	u64 phys_limit;
> >> -	void *ret;
> >>  
> >>  	if (WARN_ON_ONCE(!IS_ENABLED(CONFIG_DMA_COHERENT_POOL)))
> >>  		return NULL;
> >>  
> >>  	gfp |= dma_direct_optimal_gfp_mask(dev, &phys_limit);
> >> -	page = dma_alloc_from_pool(dev, size, &ret, gfp, dma_coherent_ok);
> >> +	page = dma_alloc_from_pool(dev, size, cpu_addr, gfp, dma_coherent_ok);
> >>  	if (!page)
> >>  		return NULL;
> >>  	*dma_handle = phys_to_dma_direct(dev, page_to_phys(page));
> >> -	return ret;
> >> +	return page;
> >>  }
> >>  
> >>  static void *dma_direct_alloc_no_mapping(struct device *dev, size_t size,
> >> @@ -247,8 +246,11 @@ void *dma_direct_alloc(struct device *dev, size_t size,
> >>  	 * the atomic pools instead if we aren't allowed block.
> >>  	 */
> >>  	if ((remap || force_dma_unencrypted(dev)) &&
> >> -	    dma_direct_use_pool(dev, gfp))
> >> -		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
> >> +	    dma_direct_use_pool(dev, gfp)) {
> >> +		page = dma_direct_alloc_from_pool(dev, size, dma_handle,
> >> +						  &ret, gfp);
> >> +		return page ? ret : NULL;
> >
> > Sorry for joining the discussion late, but the line above caught my
> > attention.
> >
> > Why do we need both ret and page? We can derive cpu_addr from page and
> > vice versa. Do we really need the &ret parameter? Or, more generally, do
> > we really need "struct page *"?
> >
> > static struct page *__dma_alloc_from_pool(struct device *dev, size_t size,
> > 		struct gen_pool *pool, void **cpu_addr,
> > 		bool (*phys_addr_ok)(struct device *, phys_addr_t, size_t))
> > {
> > ...
> > 	*cpu_addr = (void *)addr;
> > 	memset(*cpu_addr, 0, size);
> > 	return pfn_to_page(__phys_to_pfn(phys));
> > }
> >
> > Why
> >
> 
> With CONFIG_DMA_DIRECT_REMAP the cpu_addr can be different from
> page_address.

Can you please point to the code there it can happen?
__dma_alloc_from_pool() has direct connection between physical address
and struct page.

Thanks

> 
> -aneesh
>