RE: [PATCH 4/4] ipc/shm.c: Increase the defaults for SHMALL, SHMMAX.

[Motohiro Kosaki]

> -----Original Message-----
> From: Manfred Spraul [mailto:manfred@xxxxxxxxxxxxxxxx]
> Sent: Monday, April 21, 2014 10:27 AM
> To: Davidlohr Bueso; Michael Kerrisk; Martin Schwidefsky
> Cc: LKML; Andrew Morton; KAMEZAWA Hiroyuki; Motohiro Kosaki JP; gthelen@xxxxxxxxxx; aswin@xxxxxx; linux-mm@xxxxxxxxx;
> Manfred Spraul
> Subject: [PATCH 4/4] ipc/shm.c: Increase the defaults for SHMALL, SHMMAX.
> 
> System V shared memory
> 
> a) can be abused to trigger out-of-memory conditions and the standard
>    measures against out-of-memory do not work:
> 
>     - it is not possible to use setrlimit to limit the size of shm segments.
> 
>     - segments can exist without association with any processes, thus
>       the oom-killer is unable to free that memory.
> 
> b) is typically used for shared information - today often multiple GB.
>    (e.g. database shared buffers)
> 
> The current default is a maximum segment size of 32 MB and a maximum total size of 8 GB. This is often too much for a) and not
> enough for b), which means that lots of users must change the defaults.
> 
> This patch increases the default limits (nearly) to the maximum, which is perfect for case b). The defaults are used after boot and as
> the initial value for each new namespace.
> 
> Admins/distros that need a protection against a) should reduce the limits and/or enable shm_rmid_forced.
> 
> Further notes:
> - The patch only changes default, overrides behave as before:
>         # sysctl kernel.shmall=33554432
>   would recreate the previous limit for SHMMAX (for the current namespace).
> 
> - Disabling sysv shm allocation is possible with:
>         # sysctl kernel.shmall=0
>   (not a new feature, also per-namespace)
> 
> - The limits are intentionally set to a value slightly less than ULONG_MAX,
>   to avoid triggering overflows in user space apps.
>   [not unreasonable, see http://marc.info/?l=linux-mm&m=139638334330127]
> 
> Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> Reported-by: Davidlohr Bueso <davidlohr@xxxxxx>
> Cc: mtk.manpages@xxxxxxxxx

Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
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