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

[Eric B Munson]

On Wed, 13 May 2015, 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.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>

Does the problem still happend when MAP_POPULATE | MAP_LOCKED is used
(AFAICT MAP_POPULATE will cause the mmap to fail if all the pages cannot
be made present).

Either way this is a good catch.

Acked-by: Eric B Munson <emunson@xxxxxxxxxx>

Attachment: signature.asc
Description: Digital signature