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

[Michal Hocko]

If there are no objections here I will resubmit this and MAP_POPULATE
patches in few days.

On Wed 29-04-15 13:38:18, Michal Hocko wrote:
> On Tue 28-04-15 11:38:35, Linus Torvalds wrote:
> > On Tue, Apr 28, 2015 at 11:35 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> > >
> > > I am still not sure I see the problem here.
> > 
> > Basically, I absolutely hate the notion of us doing something
> > unsynchronized, when I can see us undoing a mmap that another thread
> > is doing. It's wrong.
> 
> OK, I have checked the mmap(2) man page and there is no single mention
> about multi-threaded usage. So even though I personally think that
> user fault handlers which do mmap(MAP_FIXED) without synchronization
> to parallel mmaps are broken by definition we cannot simply rule them
> out and it is not the kernel job to make them broken even more or in a
> subtly different way.
> So here is an RFC for the man page patch. I am not very good in the
> format but man doesn't complain about any formating issues.
> ---
> From 903ed733187afaa4d27fef3c24f413304494411c Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxx>
> Date: Wed, 29 Apr 2015 11:02:19 +0200
> Subject: [RFC PATCH] mmap.2: clarify MAP_LOCKED semantic
> 
> 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
> -- 
> 2.1.4
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs
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