On Wed, Feb 13, 2019 at 12:47 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > On Mon 11-02-19 17:32:03, Jann Horn wrote: > > The current manpage reads to me as if the kernel will always pick a free > > space close to the requested address, but that's not the case: > > > > mmap(0x600000000000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, > > -1, 0) = 0x600000000000 > > mmap(0x600000000000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, > > -1, 0) = 0x7f5042859000 > > > > You can also see this in the various implementations of > > ->get_unmapped_area() - if the specified address isn't available, the > > kernel basically ignores the hint (apart from the 5level paging hack). > > > > Clarify how this works a bit. > > Do we really want to be that specific? What if a future implementation > would like to ignore the mapping even if there is no colliding mapping > already? E.g. becuase of fragmentation avoidance or whatever other > reason. If we are explicit about the current implementation we might > give a receipt to userspace to depend on that behavior. You have a point. So I guess we want something like this? "If another mapping already exists there, the kernel picks a new address that may or may not depend on the hint." Unless someone can come up with a nicer wording for this?
