netdev - Re: [PATCH net-next v5 2/2] page_pool: Track DMA-mapped pages and unmap them when destroying the pool

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Message-ID: <38b9af46-0d03-424d-8ecc-461b7daf216c@redhat.com>
Date: Tue, 1 Apr 2025 10:52:17 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>,
 "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Jesper Dangaard Brouer <hawk@...nel.org>, Saeed Mahameed
 <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>,
 Tariq Toukan <tariqt@...dia.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 Eric Dumazet <edumazet@...gle.com>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>,
 Simon Horman <horms@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
 Mina Almasry <almasrymina@...gle.com>, Yonglong Liu
 <liuyonglong@...wei.com>, Yunsheng Lin <linyunsheng@...wei.com>,
 Pavel Begunkov <asml.silence@...il.com>, Matthew Wilcox <willy@...radead.org>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, linux-rdma@...r.kernel.org,
 linux-mm@...ck.org, Qiuling Ren <qren@...hat.com>,
 Yuying Ma <yuma@...hat.com>
Subject: Re: [PATCH net-next v5 2/2] page_pool: Track DMA-mapped pages and
 unmap them when destroying the pool

On 3/28/25 1:19 PM, Toke Høiland-Jørgensen wrote:
> @@ -463,13 +462,21 @@ page_pool_dma_sync_for_device(const struct page_pool *pool,
>  			      netmem_ref netmem,
>  			      u32 dma_sync_size)
>  {
> -	if (pool->dma_sync && dma_dev_need_sync(pool->p.dev))
> -		__page_pool_dma_sync_for_device(pool, netmem, dma_sync_size);
> +	if (pool->dma_sync && dma_dev_need_sync(pool->p.dev)) {

Lacking a READ_ONCE() here, I think it's within compiler's right do some
unexpected optimization between this read and the next one. Also it will
make the double read more explicit.

Thanks,

Paolo