linux-kernel - Re: [PATCH 07/32] kmsan: Remove a useless assignment from kmsan_vmap_pages_range

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Date:   Thu, 16 Nov 2023 11:52:10 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Ilya Leoshkevich <iii@...ux.ibm.com>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Marco Elver <elver@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Pekka Enberg <penberg@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH 07/32] kmsan: Remove a useless assignment from kmsan_vmap_pages_range_noflush()

On Wed, Nov 15, 2023 at 9:34 PM Ilya Leoshkevich <iii@...ux.ibm.com> wrote:
>
> The value assigned to prot is immediately overwritten on the next line
> with PAGE_KERNEL. The right hand side of the assignment has no
> side-effects.
>
> Fixes: b073d7f8aee4 ("mm: kmsan: maintain KMSAN metadata for page operations")
> Suggested-by: Alexander Gordeev <agordeev@...ux.ibm.com>
> Signed-off-by: Ilya Leoshkevich <iii@...ux.ibm.com>
Reviewed-by: Alexander Potapenko <glider@...gle.com>

> ---
>  mm/kmsan/shadow.c | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/mm/kmsan/shadow.c b/mm/kmsan/shadow.c
> index b9d05aff313e..2d57408c78ae 100644
> --- a/mm/kmsan/shadow.c
> +++ b/mm/kmsan/shadow.c
> @@ -243,7 +243,6 @@ int kmsan_vmap_pages_range_noflush(unsigned long start, unsigned long end,
>                 s_pages[i] = shadow_page_for(pages[i]);
>                 o_pages[i] = origin_page_for(pages[i]);
>         }
> -       prot = __pgprot(pgprot_val(prot) | _PAGE_NX);
>         prot = PAGE_KERNEL;

This bug dates back to 5.1-rc2, when KMSAN didn't exist upstream.
The commit introducing vmap support already had it:
https://github.com/google/kmsan/commit/3ff9d7c640d378485286e1a99d85984ae6901f23
I don't remember what exactly required the more relaxed PAGE_KERNEL
mask though :)