linux-kernel - Re: [PATCH v10] mm, hugetlbfs: Allow for "high" userspace addresses

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Message-Id: <20220415150929.a62cbad83c22d6304560f626@linux-foundation.org>
Date:   Fri, 15 Apr 2022 15:09:29 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Steve Capper <steve.capper@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v10] mm, hugetlbfs: Allow for "high" userspace addresses

On Fri, 15 Apr 2022 16:45:13 +0200 Christophe Leroy <christophe.leroy@...roup.eu> wrote:

> This is a fix for commit f6795053dac8 ("mm: mmap: Allow for "high"
> userspace addresses") for hugetlb.
> 
> This patch adds support for "high" userspace addresses that are
> optionally supported on the system and have to be requested via a hint
> mechanism ("high" addr parameter to mmap).
> 
> Architectures such as powerpc and x86 achieve this by making changes to
> their architectural versions of hugetlb_get_unmapped_area() function.
> However, arm64 uses the generic version of that function.
> 
> So take into account arch_get_mmap_base() and arch_get_mmap_end() in
> hugetlb_get_unmapped_area(). To allow that, move those two macros
> out of mm/mmap.c into include/linux/sched/mm.h
> 
> If these macros are not defined in architectural code then they default
> to (TASK_SIZE) and (base) so should not introduce any behavioural
> changes to architectures that do not define them.
> 
> For the time being, only ARM64 is affected by this change.
> 
> >From Catalin (ARM64):
>   We should have fixed hugetlb_get_unmapped_area() as well when we
> added support for 52-bit VA. The reason for commit f6795053dac8 was to
> prevent normal mmap() from returning addresses above 48-bit by default
> as some user-space had hard assumptions about this.
> 
> It's a slight ABI change if you do this for hugetlb_get_unmapped_area()
> but I doubt anyone would notice. It's more likely that the current
> behaviour would cause issues, so I'd rather have them consistent.

I'm struggling to understand the need for a -stable backport from the
above text.

Could we please get a simple statement of the end-user visible effects
of the shortcoming?  Target audience is -stable tree maintainers, and
people who we've never heard of who will be wondering whether they should
add this to their organization's older kernel.

>  fs/hugetlbfs/inode.c     | 9 +++++----
>  include/linux/sched/mm.h | 8 ++++++++
>  mm/mmap.c                | 8 --------
>  3 files changed, 13 insertions(+), 12 deletions(-)

I'm a bit surprised that this has reached version 10!  Was it really
that tricky?