On 2/14/19 5:18 PM, Jann Horn wrote: > The current manpage reads to me as if the kernel will always pick a free > space close to the requested address, but that's not the case: > > mmap(0x600000000000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, > -1, 0) = 0x600000000000 > mmap(0x600000000000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, > -1, 0) = 0x7f5042859000 > > You can also see this in the various implementations of > ->get_unmapped_area() - if the specified address isn't available, the > kernel basically ignores the hint (apart from the 5level paging hack). > > Clarify how this works a bit. > > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> Thanks, Jann. Patch applied. And thanks for the review, Michal. Cheers, Michael > --- > changed in v2: > - be less specific about what the kernel does when the requested address > is unavailable to avoid constraining future behavior changes > (Michal Hocko) > > man2/mmap.2 | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/man2/mmap.2 b/man2/mmap.2 > index fccfb9b3e..dbcae59be 100644 > --- a/man2/mmap.2 > +++ b/man2/mmap.2 > @@ -71,7 +71,12 @@ If > .I addr > is not NULL, > then the kernel takes it as a hint about where to place the mapping; > -on Linux, the mapping will be created at a nearby page boundary. > +on Linux, the kernel will pick a nearby page boundary (but always above > +or equal to the value specified by > +.IR /proc/sys/vm/mmap_min_addr ) > +and attempt to create the mapping there. > +If another mapping already exists there, the kernel picks a new address that > +may or may not depend on the hint. > .\" Before Linux 2.6.24, the address was rounded up to the next page > .\" boundary; since 2.6.24, it is rounded down! > The address of the new mapping is returned as the result of the call. > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
