linux-kernel - Re: [PATCH v4] lib/test_bpf: Call page_address() on page acquired with GFP

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Date:   Tue, 27 Jun 2023 08:01:38 -0700
From:   Sumitra Sharma <sumitraartsy@...il.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     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,
        Ira Weiny <ira.weiny@...el.com>,
        Deepak R Varma <drv@...lo.com>,
        Sumitra Sharma <sumitraartsy@...il.com>
Subject: Re: [PATCH v4] lib/test_bpf: Call page_address() on page acquired
 with GFP_KERNEL flag

On Mon, Jun 26, 2023 at 03:07:22PM +0200, Fabio M. De Francesco wrote:
> On venerdì 23 giugno 2023 17:16:44 CEST Sumitra Sharma wrote:
> > generate_test_data() acquires a page with alloc_page(GFP_KERNEL).
> > The GFP_KERNEL is typical for kernel-internal allocations.
> > The caller requires ZONE_NORMAL or a lower zone for direct access.
> > 
> > Therefore the page cannot come from ZONE_HIGHMEM. Thus there's
> > no need to map it with kmap().
> > 
> > Also, the kmap() is being deprecated in favor of kmap_local_page() [1].
> > 
> > Hence, use a plain page_address() directly.
> > 
> > Since the page passed to the page_address() is not from the highmem
> > zone, the page_address() function will always return a valid kernel
> > virtual address and will not return NULL. Hence, remove the check
> > 'if (!ptr)'.
> > 
> > Remove the unused variable 'ptr' and label 'err_free_page'.
> > 
> > [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com/
> > 
> > Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> 
> I can't see no more things that prevent my tag, then it is...
> 
> Reviewed-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> 
> Thanks for working on it for all the time it took to get to v4 and for 
> listening to what you've been suggested to change.
> 
> Well done :-)

Thank you, Fabio and Ira, for your tags and kind words :)

Thanks & regards
Sumitra

> 
> Fabio
> 
> > Signed-off-by: Sumitra Sharma <sumitraartsy@...il.com>
> > ---
> > 
> > - Link to v1:
> > https://lore.kernel.org/bpf/20230613073020.GA359792@sumitra.com/T/ - Link to
> > v2: https://lore.kernel.org/all/20230613071756.GA359746@sumitra.com/ - Link
> > to v3: https://lore.kernel.org/all/20230622080729.GA426913@sumitra.com/
> > 
> > Changes in v4:
> > Reported-by: kernel test robot <lkp@...el.com>
> > Closes:
> > https://lore.kernel.org/oe-kbuild-all/202306230559.hU5Aonpl-lkp@intel.com/ -
> > Remove unused label 'err_free_page'.
> > 
> > Changes in v3:
> > Noted by: Fabio M. De Francesco<fmdefrancesco@...il.com>
> > 	- Remove the check 'if (!ptr)'.
> > 	- Remove the unused variable 'ptr'.
> > 	- Change the commit message.
> > 
> > Changes in v2:
> > Noted by: Fabio M. De Francesco<fmdefrancesco@...il.com>
> > 	- Remove the kmap() call and call page_address() instead.
> > 	- Change the commit subject and message.
> > 
> >  lib/test_bpf.c | 10 +---------
> >  1 file changed, 1 insertion(+), 9 deletions(-)
> > 
> > diff --git a/lib/test_bpf.c b/lib/test_bpf.c
> > index ade9ac672adb..a5cc5f9fc4e8 100644
> > --- a/lib/test_bpf.c
> > +++ b/lib/test_bpf.c
> > @@ -14381,25 +14381,17 @@ static void *generate_test_data(struct bpf_test
> > *test, int sub) * single fragment to the skb, filled with
> >  		 * test->frag_data.
> >  		 */
> > -		void *ptr;
> > -
> >  		page = alloc_page(GFP_KERNEL);
> > 
> >  		if (!page)
> >  			goto err_kfree_skb;
> > 
> > -		ptr = kmap(page);
> > -		if (!ptr)
> > -			goto err_free_page;
> > -		memcpy(ptr, test->frag_data, MAX_DATA);
> > -		kunmap(page);
> > +		memcpy(page_address(page), test->frag_data, MAX_DATA);
> >  		skb_add_rx_frag(skb, 0, page, 0, MAX_DATA, MAX_DATA);
> >  	}
> > 
> >  	return skb;
> > 
> > -err_free_page:
> > -	__free_page(page);
> >  err_kfree_skb:
> >  	kfree_skb(skb);
> >  	return NULL;
> > --
> > 2.25.1
> 
> 
> 
>