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