On Wed 11-04-18 18:40:09, Jann Horn wrote: > On Wed, Apr 11, 2018 at 6:36 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > On Wed 11-04-18 17:37:46, Jann Horn wrote: > >> On Wed, Apr 11, 2018 at 2:04 PM, <mhocko@xxxxxxxxxx> wrote: > >> > From: Michal Hocko <mhocko@xxxxxxxx> > >> > > >> > 4.17+ kernels offer a new MAP_FIXED_NOREPLACE flag which allows the caller to > >> > atomicaly probe for a given address range. > >> > > >> > [wording heavily updated by John Hubbard <jhubbard@xxxxxxxxxx>] > >> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > >> > --- > >> > Hi, > >> > Andrew's sent the MAP_FIXED_NOREPLACE to Linus for the upcoming merge > >> > window. So here we go with the man page update. > >> > > >> > man2/mmap.2 | 27 +++++++++++++++++++++++++++ > >> > 1 file changed, 27 insertions(+) > >> > > >> > diff --git a/man2/mmap.2 b/man2/mmap.2 > >> > index ea64eb8f0dcc..f702f3e4eba2 100644 > >> > --- a/man2/mmap.2 > >> > +++ b/man2/mmap.2 > >> > @@ -261,6 +261,27 @@ Examples include > >> > and the PAM libraries > >> > .UR http://www.linux-pam.org > >> > .UE . > >> > +Newer kernels > >> > +(Linux 4.17 and later) have a > >> > +.B MAP_FIXED_NOREPLACE > >> > +option that avoids the corruption problem; if available, MAP_FIXED_NOREPLACE > >> > +should be preferred over MAP_FIXED. > >> > >> This still looks wrong to me. There are legitimate uses for MAP_FIXED, > >> and for most users of MAP_FIXED that I'm aware of, MAP_FIXED_NOREPLACE > >> wouldn't work while MAP_FIXED works perfectly well. > >> > >> MAP_FIXED is for when you have already reserved the targeted memory > >> area using another VMA; MAP_FIXED_NOREPLACE is for when you haven't. > >> Please don't make it sound as if MAP_FIXED is always wrong. > > > > Well, this was suggested by John. I think, nobody is objecting that > > MAP_FIXED has legitimate usecases. The above text just follows up on > > the previous section which emphasises the potential memory corruption > > problems and it suggests that a new flag is safe with that regards. > > > > If you have specific wording that would be better I am open for changes. > > I guess I'd probably also want to change the previous text; so I > should probably send a followup patch once this one has landed. yeah, that sounds like a better plan. -- Michal Hocko SUSE Labs
