Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX_VADDR

[Andy Lutomirski]

On Fri, Feb 17, 2017 at 9:19 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> On 02/17/2017 06:13 AM, Kirill A. Shutemov wrote:
>> +/*
>> + * Default maximum virtual address. This is required for
>> + * compatibility with applications that assumes 47-bit VA.
>> + * The limit can be changed with prctl(PR_SET_MAX_VADDR).
>> + */
>> +#define MAX_VADDR_DEFAULT    ((1UL << 47) - PAGE_SIZE)
>
> This is a bit goofy.  It's not the largest virtual adddress that can be
> accessed, but the beginning of the last page.

No, it really is the limit.  We don't allow user code to map the last
page because ti would be a root hole due to SYSRET.  Thanks, Intel.
See the comment near TASK_SIZE_MAX IIRC.

--Andy