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Date: Tue, 18 Jul 2023 17:40:42 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>, Eric Dumazet
 <edumazet@google.com>, Paolo Abeni <pabeni@redhat.com>, Maciej Fijalkowski
 <maciej.fijalkowski@intel.com>, Larysa Zaremba <larysa.zaremba@intel.com>,
 Yunsheng Lin <linyunsheng@huawei.com>, Alexander Duyck
 <alexanderduyck@fb.com>, Jesper Dangaard Brouer <hawk@kernel.org>, Ilias
 Apalodimas <ilias.apalodimas@linaro.org>, netdev@vger.kernel.org,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC net-next v2 7/7] net: skbuff: always try to recycle
 PP pages directly when in softirq
Message-ID: <20230718174042.67c02449@kernel.org>
In-Reply-To: <20230714170853.866018-10-aleksander.lobakin@intel.com>
References: <20230714170853.866018-1-aleksander.lobakin@intel.com>
	<20230714170853.866018-10-aleksander.lobakin@intel.com>
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On Fri, 14 Jul 2023 19:08:52 +0200 Alexander Lobakin wrote:
> Suggested-by: Jakub Kicinski <kuba@kernel.org> # in_softirq()

I thought I said something along the lines as "if this is safe you can
as well" which falls short of a suggestion, cause I don't think it is
safe :)

> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index fc1470aab5cf..1c22fd33be6c 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -902,7 +902,7 @@ bool page_pool_return_skb_page(struct page *page, bool napi_safe)
>  	 * in the same context as the consumer would run, so there's
>  	 * no possible race.
>  	 */
> -	if (napi_safe) {
> +	if (napi_safe || in_softirq()) {
>  		const struct napi_struct *napi = READ_ONCE(pp->p.napi);
>  
>  		allow_direct = napi &&

What if we got here from netpoll? napi budget was 0, so napi_safe is
false, but in_softirq() can be true or false.

XDP SKB is a toy, I really don't think 3-4% in XDP SKB warrants the
risk here.