linux-kernel - Re: [PATCH v2] lib/test_bpf: Call page_address() on page acquired with GFP

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Date:   Sun, 18 Jun 2023 07:07:00 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sumitra Sharma <sumitraartsy@...il.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH v2] lib/test_bpf: Call page_address() on page acquired with
 GFP_KERNEL flag

On martedì 13 giugno 2023 09:17:56 CEST Sumitra Sharma wrote:
> generate_test_data() acquires a page with alloc_page(GFP_KERNEL). Pages
> allocated with GFP_KERNEL cannot come from Highmem. This is why
> there is no need to call kmap() on them.
> 
> Therefore, use a plain page_address() on that page.
> 
> Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Signed-off-by: Sumitra Sharma <sumitraartsy@...il.com>
> ---
> 
> Changes in v2:
> 	- Remove the kmap() call and call page_address() instead.

NIT: Give credit to whom asked you for this removal and explain why the 
mapping is not required.

> 	- Change the commit subject and message.
> 
>  lib/test_bpf.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/lib/test_bpf.c b/lib/test_bpf.c
> index ade9ac672adb..70fcd0bcf14b 100644
> --- a/lib/test_bpf.c
> +++ b/lib/test_bpf.c
> @@ -14388,11 +14388,10 @@ static void *generate_test_data(struct bpf_test
> *test, int sub) if (!page)
>  			goto err_kfree_skb;
> 
> -		ptr = kmap(page);
> +		ptr = page_address(page);
>  		if (!ptr)
>  			goto err_free_page;

What is the reason of this test? Could "ptr" ever be NULL? What is the code 
checking just few lines above this latter test?

Please, take a deeper look at this function as a whole.

Fabio

>  		memcpy(ptr, test->frag_data, MAX_DATA);
> -		kunmap(page);
>  		skb_add_rx_frag(skb, 0, page, 0, MAX_DATA, MAX_DATA);
>  	}
> 
> --
> 2.25.1