linux-kernel - Re: [PATCH] lib/test_bpf: Replace kmap() with kmap_local

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Date:   Tue, 13 Jun 2023 00:30:20 -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>
Subject: Re: [PATCH] lib/test_bpf: Replace kmap() with kmap_local_page()

On Mon, Jun 12, 2023 at 01:49:56PM +0200, Fabio M. De Francesco wrote:
> On lunedì 12 giugno 2023 12:33:41 CEST Sumitra Sharma wrote:
> > kmap() has been deprecated in favor of the kmap_local_page()
> > due to high cost, restricted mapping space, the overhead of
> > a global lock for synchronization, and making the process
> > sleep in the absence of free slots.
> > 
> > kmap_local_page() is faster than kmap() and offers thread-local
> > and CPU-local mappings, take pagefaults in a local kmap region
> > and preserves preemption by saving the mappings of outgoing
> > tasks and restoring those of the incoming one during a context
> > switch.
> > 
> > The mapping is kept thread local in the function
> > “generate_test_data” in test_bpf.c
> > 
> > Therefore, replace kmap() with kmap_local_page() and use
> > memcpy_to_page() to avoid open coding kmap_local_page()
> > + memcpy() + kunmap_local().
> > 
> > Remove the unused variable “ptr”.
> 
> Sumitra,
> 
> Please Cc your mentors while in the internship.
> It's not mandatory but it would expedite comments and reviews :-)
> 

Hi Fabio,

I will take care of it.

> > 
> > Signed-off-by: Sumitra Sharma <sumitraartsy@...il.com>
> > ---
> >  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..3bb94727d83b 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_to_page(page, 0, test->frag_data, MAX_DATA);
> 
> Why are you temporary mapping a page allocated with the GFP_KERNEL flag? 
> It cannot come from ZONE_HIGHMEM. 
>

Fabio, I somewhere _wrongly_ read that "GFP_KERNEL may allocate pages from
highmem". However, with further readings and checking the GFP_KERNEL
definition in gfp_types.h

#define GFP_KERNEL	(__GFP_RECLAIM | __GFP_IO | __GFP_FS)

I have got that with the GFP_KERNEL flag, pages can never come 
from ZONE_HIGHMEM.

Apologise.

Thanks & regards
Sumitra

> Fabio
> 
> >  		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
> 
> 
>