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/ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>