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