On Thu, 2013-07-18 at 13:29 -0700, Dave Hansen wrote: > On 07/18/2013 01:10 PM, Toshi Kani wrote: > > On Thu, 2013-07-18 at 11:34 -0700, Dave Hansen wrote: > > I do not think so. Using echo command to write a value to /dev/sda is > > not how it is instructed to use in the document. I am not saying that > > we need to protect from a privileged user doing something crazy. > > If the document is the issue, then let's fix the document. I will clarify the document as well. > >> All that I'm asking is that you either leave it the way it is, or make a > >> Kconfig menu entry for it. > >> > >> But, really, what's the problem that you're solving? Has this caused > >> you issues somehow? It's been there for, what, 10 years? Surely it's > >> part of the ABI. > > > > The problem is that the probe interface is documented as one of the > > steps that may be necessary for memory hotplug. A typical user may or > > may not know if his/her platform generates a hotplug notification to the > > kernel to decide if this step is necessary. > > A typical user will never see any of this stuff. It's buried deep under > the covers. Users will need to use sysfs "memoryX/online" interface to online hot-added memory, which is located in the same directory as "probe". The name "probe" is also misleading that one would expect it checks if a given memory address is present on the system. > > If the user performs this > > step on x86, it will likely mess up the system. Since we do not need it > > on x86, a prudent approach to protect such user is to disable or remove > > the interface on x86 and document it accordingly. We have not seen this > > issue yet because we do not have many platforms that support memory > > hotplug today. Once memory hotplug support in KVM gets merged into the > > mainline, anyone can start using this feature on their systems. At that > > time, their choice of a stable kernel may be 3.12.x. This interface has > > been there for while, but we need to fix it before the memory hotplug > > feature becomes available for everyone. > > It sounds like you're arguing that anyone using memory hotplug on x86 > might be confused by the probe file. There's been a lot of hardware out > there that's supported memory hotplug for many, many years. I've never > heard a complaint about it in practice. Are KVM users more apt to be > confused than folks running on bare-metal? :) I know ia64 (which Kconfig does not have this option) and powerpc (which this interface is required, but I believe it is well-automated by LPAR mgmt tools) platforms support memory hotplug for many years, but I did not know it is also the case with x86 platforms. The fact that ia64 does not have this interface makes me wonder why we enabled it on x86. > > Does it make sense? I understand that you are using this interface for > > your testing. If I make a Kconfig menu entry, are you OK to disable > > this option by default? > > I kinda wish you wouldn't mess with it. But, sure, put it in the memory > debugging, and make sure it stays enabled on powerpc by default. I understand and I appreciate your flexibility. Since it is defined in x86/Kconfig, the prompt will be under "Processor type and features". Yes, it stays enabled on powerpc as I won't touch powerpc/Kconfig. > Another method would be to just change the default permissions of the > file on x86 instead of disabling it completely: > > # chmod u-w /sys/devices/system/memory/probe > # echo x > /sys/devices/system/memory/probe > bash: /sys/devices/system/memory/probe: Permission denied > > That way folks can re-chmod it if they *really* want it back (me), and > they can still use it for testing. That's an interesting idea, but I'd prefer not to introduce #ifdef to the common code for this. Thanks, -Toshi -- 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>