Re: [PATCH 1/2] mmap.2: clarify MAP_LOCKED semantic

From: Michael Kerrisk (man-pages)
Date: Thu May 14 2015 - 09:36:20 EST

On 05/13/2015 04:38 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxx>
> 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.

Thanks, Michal. Applied, with Reviewed-by: from Eric added.



> 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
> +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

Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:
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