Re: [PATCH] ipc,shm: increase default size for shmmax

[Michael Kerrisk (man-pages)]

On Thu, Apr 17, 2014 at 6:41 PM, Manfred Spraul
<manfred@xxxxxxxxxxxxxxxx> wrote:
> On 04/17/2014 12:41 PM, Michael Kerrisk wrote:
>>
>> On Thu, Apr 17, 2014 at 12:46 AM, Andrew Morton
>> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>> On Sun, 13 Apr 2014 20:05:34 +0200 Manfred Spraul
>>> <manfred@xxxxxxxxxxxxxxxx> wrote:
>>>
>>>> Hi Andrew,
>>>>
>>>> On 04/02/2014 12:08 AM, Andrew Morton wrote:
>>>>>
>>>>> Well, I'm assuming 64GB==infinity. It *was* infinity in the RHEL5
>>>>> timeframe, but infinity has since become larger so pickanumber.
>>>>
>>>> I think infinity is the right solution:
>>>> The only common case where infinity is wrong would be Android - and
>>>> Android disables sysv shm entirely.
>>>>
>>>> There are two patches:
>>>> http://marc.info/?l=linux-kernel&m=139730332306185&q=raw
>>>> http://marc.info/?l=linux-kernel&m=139727299800644&q=raw
>>>>
>>>> Could you apply one of them?
>>>> I wrote the first one, thus I'm biased which one is better.
>>>
>>> I like your patch because applying it might encourage you to send more
>>> kernel patches - I miss the old days ;)
>>>
>>> But I do worry about disrupting existing systems so I like Davidlohr's
>>> idea of making the change a no-op for people who are currently
>>> explicitly setting shmmax and shmall.
>>
>> Agreed. It's hard to imagine situations where people might care
>> nowadays, but there's no limits to people's insane inventiveness. Some
>> people really might want to set an upper limit.
>
> I don't understand that: neither patch has any impact after an explicit
> sysctl that overwrites shmmax.

You don't understand it, because I was being dense :-}. I
misunderstood your patch. I think I was thrown by this line in the
commit message:

    The patch disables both limits by setting the limits to ULONG_MAX.

Of course, you patch doesn't *disable* the limits, it simply sets the
defaults to the maximum.

>>> In an ideal world, system administrators would review this change,
>>
>> And in the ideal world, patches such as this would CC
>> linux-api@xxxxxxxxxxxxxxx, as described in
>> Documentation/SubmitChecklist, so that users who care about getting
>> advance warning on API changes could be alerted and might even review
>> and comment...
>
> Good point.
> Davidlohr: Your patch has an impact on shmctl(,IPC_INFO,).
> Could you add that for v3?

Well, actually, BOTH patches change the API, because they both affect
SHMALL/SHMMAX.

Cheers,

Michael


> I'll try to make a v2 (with your update to the uapi header file) tomorrow.
>
> --
>     Manfred



-- 
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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