Re: [PATCH] ipc,shm: disable shmmax and shmall by default

[Davidlohr Bueso]

On Thu, 2014-04-03 at 21:02 +0200, Manfred Spraul wrote:
> Hi Davidlohr,
> 
> On 04/03/2014 02:20 AM, Davidlohr Bueso wrote:
> > The default size for shmmax is, and always has been, 32Mb.
> > Today, in the XXI century, it seems that this value is rather small,
> > making users have to increase it via sysctl, which can cause
> > unnecessary work and userspace application workarounds[1].
> >
> > Instead of choosing yet another arbitrary value, larger than 32Mb,
> > this patch disables the use of both shmmax and shmall by default,
> > allowing users to create segments of unlimited sizes. Users and
> > applications that already explicitly set these values through sysctl
> > are left untouched, and thus does not change any of the behavior.
> >
> > So a value of 0 bytes or pages, for shmmax and shmall, respectively,
> > implies unlimited memory, as opposed to disabling sysv shared memory.
> > This is safe as 0 cannot possibly be used previously as SHMMIN is
> > hardcoded to 1 and cannot be modified.

> Are we sure that no user space apps uses shmctl(IPC_INFO) and prints a 
> pretty error message if shmall is too small?
> We would break these apps.

Good point. 0 bytes/pages would definitely trigger an unexpected error
message if users did this. But on the other hand I'm not sure this
actually is a _real_ scenario, since upon overflow the value can still
end up being 0, which is totally bogus and would cause the same
breakage.

So I see two possible workarounds:
(i) Use ULONG_MAX for the shmmax default instead. This would make shmall
default to 1152921504606846720 and 268435456, for 64 and 32bit systems,
respectively.

(ii) Keep the 0 bytes, but add a new a "transition" tunable that, if set
(default off), would allow 0 bytes to be unlimited. With time, users
could hopefully update their applications and we could eventually get
rid of it. This _seems_ to be the less aggressive way to go.

Thoughts?

Thanks,
Davidlohr

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