If there are no objections here I will resubmit this and MAP_POPULATE patches in few days. On Wed 29-04-15 13:38:18, Michal Hocko wrote: > On Tue 28-04-15 11:38:35, Linus Torvalds wrote: > > On Tue, Apr 28, 2015 at 11:35 AM, Michal Hocko <mhocko@xxxxxxx> wrote: > > > > > > I am still not sure I see the problem here. > > > > Basically, I absolutely hate the notion of us doing something > > unsynchronized, when I can see us undoing a mmap that another thread > > is doing. It's wrong. > > OK, I have checked the mmap(2) man page and there is no single mention > about multi-threaded usage. So even though I personally think that > user fault handlers which do mmap(MAP_FIXED) without synchronization > to parallel mmaps are broken by definition we cannot simply rule them > out and it is not the kernel job to make them broken even more or in a > subtly different way. > So here is an RFC for the man page patch. I am not very good in the > format but man doesn't complain about any formating issues. > --- > From 903ed733187afaa4d27fef3c24f413304494411c Mon Sep 17 00:00:00 2001 > From: Michal Hocko <mhocko@xxxxxxx> > Date: Wed, 29 Apr 2015 11:02:19 +0200 > Subject: [RFC PATCH] mmap.2: clarify MAP_LOCKED semantic > > 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> > --- > man2/mmap.2 | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/man2/mmap.2 b/man2/mmap.2 > index 54d68cf87e9e..1486be2e96b3 100644 > --- a/man2/mmap.2 > +++ b/man2/mmap.2 > @@ -235,8 +235,19 @@ See the Linux kernel source file > for further information. > .TP > .BR MAP_LOCKED " (since Linux 2.5.37)" > -Lock the pages of the mapped region into memory in the manner of > +Mark the mmaped region to be locked in the same way as > .BR mlock (2). > +This implementation will try to populate (prefault) the whole range but > +the mmap call doesn't fail with > +.B ENOMEM > +if this fails. Therefore major faults might happen later on. So the semantic > +is not as strong as > +.BR mlock (2). > +.BR mmap (2) > ++ > +.BR mlock (2) > +should be used when major faults are not acceptable after the initialization > +of the mapping. > This flag is ignored in older kernels. > .\" If set, the mapped pages will not be swapped out. > .TP > -- > 2.1.4 > > -- > Michal Hocko > SUSE Labs -- 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>