Re: [RFC PATCH] mmap.2: clarify MAP_LOCKED semantic (was: Re: Should mmap MAP_LOCKED fail if mm_poppulate fails?)

[David Rientjes]

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 <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

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?
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