Messages in this thread | | | Date | Wed, 24 Jul 2013 08:18:00 +0800 | From | Hush Bensen <> | Subject | Re: [PATCH v2] mm/hotplug, x86: Disable ARCH_MEMORY_PROBE by default |
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On 07/24/2013 04:45 AM, Toshi Kani wrote: > On Tue, 2013-07-23 at 10:01 +0200, Ingo Molnar wrote: >> * Toshi Kani <toshi.kani@hp.com> wrote: >> >>>> Could we please also fix it to never crash the kernel, even if stupid >>>> ranges are provided? >>> Yes, this probe interface can be enhanced to verify the firmware >>> information before adding a given memory address. However, such change >>> would interfere its test use of "fake" hotplug, which is only the known >>> use-case of this interface on x86. >> Not crashing the kernel is not a novel concept even for test interfaces... > Agreed. > >> Where does the possible crash come from - from using invalid RAM ranges, >> right? I.e. on x86 to fix the crash we need to check the RAM is present in >> the e820 maps, is marked RAM there, and is not already registered with the >> kernel, or so? > Yes, the crash comes from using invalid RAM ranges. How to check if the > RAM is present is different if the system supports hotplug or not.
Could you explain different methods to check the RAM is present if the system supports hotplkug or not?
> >>> In order to verify if a given memory address is enabled at run-time (as >>> opposed to boot-time), we need to check with ACPI memory device objects >>> on x86. However, system vendors tend to not implement memory device >>> objects unless their systems support memory hotplug. Dave Hansen is >>> using this interface for his testing as a way to fake a hotplug event on >>> a system that does not support memory hotplug. >> All vendors implement e820 maps for the memory present at boot time. > Yes for boot time. At run-time, e820 is not guaranteed to represent a > new memory added. Here is a quote from ACPI spec. > > === > 15.1 INT 15H, E820H - Query System Address Map > : > The memory map conveyed by this interface is not required to reflect any > changes in available physical memory that have occurred after the BIOS > has initially passed control to the operating system. For example, if > memory is added dynamically, this interface is not required to reflect > the new system memory configuration. > === > > By definition, the "probe" interface is used for the kernel to recognize > a new memory added at run-time. So, it should check ACPI memory device > objects (which represents run-time state) for the verification. On x86, > however, ACPI also sends a hotplug event to the kernel, which triggers > the kernel to recognize the new physical memory properly. Hence, users > do not need this "probe" interface. > >> How is the testing done by Dave Hansen? If it's done by booting with less >> RAM than available (via say the mem=1g boot parameter), and then >> hot-adding some of the missing RAM, then this could be made safe via the >> e820 maps and by consultig the physical memory maps (to avoid double >> registry), right? > If we focus on this test scenario on a system that does not support > hotplug, yes, I agree that we can check with e820 since it is safe to > assume that the system has no change after boot. IOW, it is unsafe to > check with e820 if the system supports hotplug, but there is no use in > this interface for testing if the system supports hotplug. So, this may > be a good idea. > > Dave, is this how you are testing? Do you always specify a valid memory > address for your testing? > >> How does the hotplug event based approach solve double adds? Relies on the >> hardware not sending a hot-add event twice for the same memory area or for >> an invalid memory area, or does it include fail-safes and double checks as >> well to avoid double adds and adding invalid memory? If yes then that >> could be utilized here as well. > In high-level, here is how ACPI memory hotplug works: > > 1. ACPI sends a hotplug event to a new ACPI memory device object that is > hot-added. > 2. The kernel is notified, and verifies if the new memory device object > has not been attached by any handler yet. > 3. The memory handler is called, and obtains a new memory range from the > ACPI memory device object. > 4. The memory handler calls add_memory() with the new address range. > > The above step 1-4 proceeds automatically within the kernel. No user > input (nor sysfs interface) is necessary. Step 2 prevents double adds > and step 3 gets a valid address range from the firmware directly. Step > 4 is basically the same as the "probe" interface, but with all the > verification up front, this step is safe.
This is hot-added part, could you also explain how ACPI memory hotplug works for hot-remove?
> > Thanks, > -Toshi > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
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