On Thu, 2014-04-17 at 22:23 +0200, Michael Kerrisk (man-pages) wrote: > 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. And it is a fair point. However, this is my counter argument: if users are checking shmmax then they sure better be checking shmmin as well! So if my patch causes shmctl(,IPC_INFO,) to return shminfo.shmmax = 0 and a user only checks this value and breaks the application, then *he's* doing it wrong. Checking shmmin is just as important... 0 value is *bogus*, heck it even says so in shmctl's manpage. > Did > one of you not yet manage to persuade the other to his point of view > yet? I think we've left that up to akpm. -- 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>