On Wed 13-05-15 10:45:06, Eric B Munson wrote: > 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). No, there is no difference because MAP_POPULATE is implicit when MAP_LOCKED is used and as pointed in the cover, we cannot fail after the vma is created and locks dropped. The second patch tries to clarify that MAP_POPULATE is just a best effort. > Either way this is a good catch. > > Acked-by: Eric B Munson <emunson@xxxxxxxxxx> Thanks! -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>