On Thu, Nov 06, 2025 at 12:31:28PM +0100, Christophe Leroy wrote:

On 32 bits it is more tricky. In theory user space can go up to
0xbfffffff while kernel will usually start at 0xc0000000. So a gap
needs to be added in-between. Allthough in theory a single 4k page
would suffice, it is easier and more efficient to enforce a 128k gap
below kernel, as it simplifies the masking.

Do we have the requirement that the first access of a masked pointer is
within 4k of the initial address?

Suppose the pointer is to an 16k array, and the memcpy happens to like
going backwards. Then a 4k hole just won't do.