On Wed, 29 Apr 2015, Michal Hocko wrote: > 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 <mho...@suse.cz> > --- > 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
The wording of this begs the question on the behavior of MAP_LOCKED | MAP_POPULATE since this same man page specifies that accesses to memory mapped with MAP_POPULATE will not block on page faults later. I think Documentation/vm/unevictable-lru.txt would benefit from an update under the mmap(MAP_LOCKED) section where all this can be laid out and perhaps reference it from the man page? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
