Re: [PATCH net-next v10 4/4] page_pool: skip dma sync operation for inflight pages

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2025/3/6 1:29, Jason Gunthorpe wrote:
> On Wed, Feb 26, 2025 at 07:03:39PM +0800, Yunsheng Lin wrote:
>> Skip dma sync operation for inflight pages before the
>> sync operation in page_pool_item_unmap() as DMA API
>> expects to be called with a valid device bound to a
>> driver as mentioned in [1].
>>
>> After page_pool_destroy() is called, the page is not
>> expected to be recycled back to pool->alloc cache and
>> dma sync operation is not needed when the page is not
>> recyclable or pool->ring is full, so only skip the dma
>> sync operation for the infilght pages by clearing the
>> pool->dma_sync, as rcu sync operation in
>> page_pool_destroy() is paired with rcu lock in
>> page_pool_recycle_in_ring() to ensure that there is no
>> dma sync operation called after rcu sync operation.
> 
> Are you guaranteeing that the cache is made consistent before freeing
> the page back to the mm? That is required..

As page_pool is only calling the sync_for_device API before the
device triggers the DMA, and the above skip is only done for the
sync_for_device API after page_pool_destroy() is called, which
means there is no DMA messing with the page before freeing the
page back to the mm if I understand the question correctly.

And the driver is supposed to call sync_for_cpu API before
passing the page to networking stack, which passes the page
back to page_pool eventually.

> 
> Jason