Re: [PATCH v7 04/22] dma: free atomic pool pages by physical address

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Jason Gunthorpe <jgg@ziepe.ca> writes:

> On Wed, Jul 01, 2026 at 11:19:08AM +0530, Aneesh Kumar K.V (Arm) wrote:
>> dma_direct_alloc_pages() may satisfy atomic allocations from the coherent
>> atomic pools. The pool allocation is keyed by the virtual address stored in
>> the gen_pool, but the pages API returns only the backing struct page.
>> 
>> On architectures with CONFIG_DMA_DIRECT_REMAP, atomic pool chunks are added
>> to the gen_pool using their remapped virtual address.
>> dma_direct_free_pages() reconstructs a linear-map address with
>> page_address(page) and passes that to dma_free_from_pool(). That address
>> does not match the gen_pool virtual range, so the pool lookup can fail and
>> the code can fall through to freeing a pool-owned page through the normal
>> page allocator path.
>> 
>> Add a page-based pool free helper that looks up the owning pool chunk by
>> physical address, translates it back to the gen_pool virtual address, and
>> frees that address to the pool. Use it from dma_direct_free_pages() while
>> keeping the existing virtual-address helper for coherent allocation frees.
>> 
>> Tested-by: Michael Kelley <mhklinux@outlook.com>
>> Tested-by: Mostafa Saleh <smostafa@google.com>
>> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>> ---
>>  include/linux/dma-map-ops.h |  1 +
>>  kernel/dma/direct.c         |  4 +--
>>  kernel/dma/pool.c           | 54 +++++++++++++++++++++++++++++++++++++
>>  3 files changed, 57 insertions(+), 2 deletions(-)
>
> This seems pretty suboptimal?
>
> If !CONFIG_DMA_DIRECT_REMAP then page_to_virt() was used to compute
> the genpool's addr so dma_free_from_pool_page() can use the same
> logic, which is how things must be working at all today
>
> The CONFIG_DMA_DIRECT_REMAP scenario does look broken, so I'm
> surprised there isn't a Fixes line on this commit? I don't have an
> opinion on the search, but since alloc_pages() is used there is 8
> bytes in the struct page that could be used to store the remapped
> vaddr to avoid the search if someday someone wants to improve
> this. Maybe a small comment hinting that direction would be a nice
> addition.
>
> Jason
>

Something like

+/*
+ * FIXME!! We could avoid this by storing the remapped virtual address in
+ * struct page and using that for lookup.
+ */
 bool dma_free_from_pool_page(struct device *dev, struct page *page, size_t size)

-aneesh