Re: [PATCH 1/2] mmap.2: clarify MAP_LOCKED semantic

[Peter Zijlstra]

On 05/13/2015 04:38 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxx>
>
> MAP_LOCKED had a subtly different semantic from mmap(2)+mlock(2) since
> it has been introduced.
> mlock(2) fails if the memory range cannot get populated to guarantee
> that no future major faults will happen on the range. mmap(MAP_LOCKED) on
> the other hand silently succeeds even if the range was populated only
> partially.
>
> Fixing this subtle difference in the kernel is rather awkward because
> the memory population happens after mm locks have been dropped and so
> the cleanup before returning failure (munlock) could operate on something
> else than the originally mapped area.
>
> E.g. speculative userspace page fault handler catching SEGV and doing
> mmap(fault_addr, MAP_FIXED|MAP_LOCKED) might discard portion of a racing
> mmap and lead to lost data. Although it is not clear whether such a
> usage would be valid, mmap page doesn't explicitly describe requirements
> for threaded applications so we cannot exclude this possibility.
>
> This patch makes the semantic of MAP_LOCKED explicit and suggest using
> mmap + mlock as the only way to guarantee no later major page faults.

URGH, this really blows chunks. It basically means MAP_LOCKED is 
pointless cruft and we might as well remove it.

Why not fix it proper?