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mmap.2: Fix description of treatment of the hint 

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.

Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Jann Horn 2019-02-14 17:18:36 +01:00 committed by Michael Kerrisk
parent c4b7b812d3
commit f58e9ed092
1 changed files with 6 additions and 1 deletions
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7
man2/mmap.2
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@ -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.