On Wed, 2013-07-24 at 08:18 +0800, Hush Bensen wrote: > 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@xxxxxx> 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? e820 and UEFI memory descriptor tables are the boot-time interfaces. These interfaces are not required to reflect any run-time changes. ACPI memory device objects can be used at both boot-time and run-time, which reflect any run-time changes. But they are optional to implement. They typically are not implemented unless the system supports hotplug. > >>> 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? Sure. Here is high-level. 1. ACPI sends a hotplug event to an ACPI memory device object that is requested to hot-remove. 2. The kernel is notified, and verifies if the memory device object is attached by a handler. 3. The memory handler is called (which is being attached), and obtains its memory range. 4. The memory handler calls remove_memory() with the address range. 5. The kernel calls eject method of the ACPI memory device object. 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/ . 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