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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue,  8 Jan 2019 12:09:44 +0100 Roman Penyaev <rpenyaev@suse.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?