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

[Davidlohr Bueso]

On Thu, 2014-04-03 at 19:39 -0400, KOSAKI Motohiro wrote:
> On Thu, Apr 3, 2014 at 3:50 PM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> > 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.
> 
> Do you mean
> 
> set 0: IPC_INFO return shmmax = 0.
> set 1: IPC_INFO return shmmax = ULONG_MAX.
> 
> ?
> 
> That makes sense.

Well I was mostly referring to:

set 0: leave things as there are now.
set 1: this patch.

I don't think it makes much sense to set unlimited for both 0 and
ULONG_MAX, that would probably just create even more confusion. 

But then again, we shouldn't even care about breaking things with shmmax
or shmall with 0 value, it just makes no sense from a user PoV. shmmax
cannot be 0 unless there's an overflow, which voids any valid cases, and
thus shmall cannot be 0 either as it would go against any values set for
shmmax. I think it's safe to ignore this.


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