linux-kernel - Re: [PATCH] xen: gntalloc.c: Convert kmap() to kmap_local

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Message-ID: <alpine.DEB.2.22.394.2204180910320.2860@hadrien>
Date:   Mon, 18 Apr 2022 09:11:27 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Alaa Mohamed <eng.alaamohamedsoliman.am@...il.com>
cc:     outreachy@...ts.linux.dev, boris.ostrovsky@...cle.com,
        jgross@...e.com, sstabellini@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        ira.weiny@...el.com
Subject: Re: [PATCH] xen: gntalloc.c:  Convert kmap() to kmap_local_page()



On Mon, 18 Apr 2022, Alaa Mohamed wrote:

> The use of kmap() is being deprecated in favor of kmap_local_page()
> where it is feasible.
>
> With kmap_local_page(), the mapping is per thread, CPU local and not
> globally visible. Therefore __del_gref() is a function
> where the use of kmap_local_page() in place of kmap() is correctly
> suited.

What is it about __del_gref() that makes this change the right choice?

julia

>
> Signed-off-by: Alaa Mohamed <eng.alaamohamedsoliman.am@...il.com>
> ---
>  drivers/xen/gntalloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/xen/gntalloc.c b/drivers/xen/gntalloc.c
> index 4849f94372a4..55acb32842a3 100644
> --- a/drivers/xen/gntalloc.c
> +++ b/drivers/xen/gntalloc.c
> @@ -178,9 +178,9 @@ static void __del_gref(struct gntalloc_gref *gref)
>  	unsigned long addr;
>
>  	if (gref->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
> -		uint8_t *tmp = kmap(gref->page);
> +		uint8_t *tmp = kmap_local_page(gref->page);
>  		tmp[gref->notify.pgoff] = 0;
> -		kunmap(gref->page);
> +		kunmap_local(tmp);
>  	}
>  	if (gref->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
>  		notify_remote_via_evtchn(gref->notify.event);
> --
> 2.35.2
>
>
>