Re: [PATCH v2] ipc,shm: disable shmmax and shmall by default

[Michael Kerrisk (man-pages)]

Hi Manfred!

On Thu, Apr 17, 2014 at 6:22 PM, Manfred Spraul
<manfred@xxxxxxxxxxxxxxxx> wrote:
> Hi Michael,
>
>
> On 04/17/2014 12:53 PM, Michael Kerrisk wrote:
>>
>> On Sat, Apr 12, 2014 at 5:22 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
>>>
>>> From: Davidlohr Bueso <davidlohr@xxxxxx>
>>>
>>> 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.
>>>
>>> This change allows Linux to treat shm just as regular anonymous memory.
>>> One important difference between them, though, is handling out-of-memory
>>> conditions: as opposed to regular anon memory, the OOM killer will not
>>> free the memory as it is shm, allowing users to potentially abuse this.
>>> To overcome this situation, the shm_rmid_forced option must be enabled.
>>>
>>> [1]: http://rhaas.blogspot.com/2012/06/absurd-shared-memory-limits.html
>>>
>>> Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
>>> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>>> Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
>>
>> Of the two proposed approaches (the other being
>> marc.info/?l=linux-kernel&m=139730332306185), this looks preferable to
>> me, since it allows strange users to maintain historical behavior
>> (i.e., the ability to set a limit) if they really want it, so:
>>
>> Acked-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
>>
>> One or two comments below, that you might consider for your v3 patch.
>
> I don't understand what you mean.

As noted in the other mail, you don't understand, because I was being
dense (and misled a little by the commit message).

> After a
>     # echo 33554432 > /proc/sys/kernel/shmmax
>     # echo 2097152 > /proc/sys/kernel/shmmax
>
> both patches behave exactly identical.

Yes.

> There are only two differences:
> - Davidlohr's patch handles
>     # echo <really huge number that doesn't fit into 64-bit> >
> /proc/sys/kernel/shmmax
>    With my patch, shmmax would end up as 0 and all allocations fail.
>
> - My patch handles the case if some startup code/installer checks
>    shmmax and complains if it is below the requirement of the application.

Thanks for that clarification. I withdraw my Ack. In fact, maybe I
even like your approach a little more, because of that last point. Did
one of you not yet manage to persuade the other to his point of view
yet?

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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