On 01/13/2014 07:33 PM, Toshi Kani wrote: > On Mon, 2014-01-13 at 18:39 -0500, Prarit Bhargava wrote: >> >> On 01/13/2014 03:31 PM, KOSAKI Motohiro wrote: >>> On Sun, Jan 12, 2014 at 6:46 PM, Prarit Bhargava <prarit@xxxxxxxxxx> wrote: >>>> >>>> >>>> On 01/11/2014 11:35 AM, 7eggert@xxxxxx wrote: >>>>> >>>>> >>>>> On Fri, 10 Jan 2014, Prarit Bhargava wrote: >>>>> >>>>>> kdump uses memmap=exactmap and mem=X values to configure the memory >>>>>> mapping for the kdump kernel. If memory is hotadded during the boot of >>>>>> the kdump kernel it is possible that the page tables for the new memory >>>>>> cause the kdump kernel to run out of memory. >>>>>> >>>>>> Since the user has specified a specific mapping ACPI Memory Hotplug should be >>>>>> disabled in this case. >>>>> >>>>> I'll ask just in case: Is it possible to want memory hotplug in spite of >>>>> using memmap=exactmap or mem=X? >>>> >>>> Good question -- I can't think of a case. When a user specifies "memmap" or >>>> "mem" IMO they are asking for a very specific memory configuration. Having >>>> extra memory added above what the user has specified seems to defeat the purpose >>>> of "memmap" and "mem". >>> >>> May be yes, may be no. >>> >>> They are often used for a wrokaround to avoid broken firmware issue. >>> If we have no way >>> to explicitly enable hotplug. We will lose a workaround. >>> >>> Perhaps, there is no matter. Today, memory hotplug is only used on >>> high-end machine >>> and their firmware is carefully developped and don't have a serious >>> issue almostly. Though. >> >> Oof -- sorry Kosaki :( I didn't see this until just now (and your subsequent >> ACK on the updated patch). >> >> I just remembered that we did have a processor vendor's whitebox that would not >> boot unless we specified a specific memmap and we did specify memmap=exactmap to >> boot the system correctly and the system had hotplug memory. >> >> So it means that I should not key off of "memmap=exactmap". > > I do not think it makes sense. You needed memmap=exactmap as a > workaround because the kernel did not boot with the firmware's memory > info. So, it's broken, and you requested the kernel to ignore the > firmware info. > > Why do you think memory hotplug needs to be supported under such > condition, which has to use the broken firmware info? There was a memory address region that was not in the e820 map that had to be reserved for a specific device's use. It was not so we used memmap=exactmap and other memmap entries to "rewrite" the e820 map to see if the system would boot; then I filed a bug against the hardware vendor ;) So, yes, in that case I did want memory hotplug & memmap=exactmap. Admittedly this was a rare corner case, and I certainly could have recompiled the kernel. P. > > 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>