linux-kernel - Re: [PATCH 1/1] mm/vmalloc: Make vmalloc_32_user() align base kernel virtual address to SHMLBA

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Date:   Tue, 8 Jan 2019 11:36:03 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Michal Hocko <mhocko@...e.com>,
        "David S . Miller" <davem@...emloft.net>,
        Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/vmalloc: Make vmalloc_32_user() align base
 kernel virtual address to SHMLBA

On Tue,  8 Jan 2019 12:09:44 +0100 Roman Penyaev <rpenyaev@...e.de> wrote:

> This patch repeats the original one from David S. Miller:
> 
>   2dca6999eed5 ("mm, perf_event: Make vmalloc_user() align base kernel virtual address to SHMLBA")
> 
> but for missed vmalloc_32_user() case, which also requires correct
> alignment of virtual address on kernel side to avoid D-caches
> aliases.  A bit of copy-paste from original patch to recover in
> memory of what is all about:
> 
>   When a vmalloc'd area is mmap'd into userspace, some kind of
>   co-ordination is necessary for this to work on platforms with cpu
>   D-caches which can have aliases.
> 
>   Otherwise kernel side writes won't be seen properly in userspace
>   and vice versa.
> 
>   If the kernel side mapping and the user side one have the same
>   alignment, modulo SHMLBA, this can work as long as VM_SHARED is
>   shared of VMA and for all current users this is true.  VM_SHARED
>   will force SHMLBA alignment of the user side mmap on platforms with
>   D-cache aliasing matters.

What are the user-visible runtime effects of this change?

Is a -stable backport needed?