On Mon, Mar 12, 2018 at 11:57:27AM +0100, Ingo Molnar wrote: > >* Baoquan He <b...@redhat.com> wrote: > >> Hi Ingo, >> >> On 03/12/18 at 10:35am, Ingo Molnar wrote: >> > >> > * Chao Fan <fanc.f...@cn.fujitsu.com> wrote: >> > >> > > Long time no reply, rebase the patchset, change the parameter name >> > > from 'kaslr_mem' to 'kaslr_boot_mem'. There's no more code change. >> > > >> > > ***Background: >> > > People reported that kaslr may randomly chooses some positions >> > > which are located in movable memory regions. This will break memory >> > > hotplug feature. >> > >> > [...] >> > >> > > ***Solutions: >> > > Introduce a new kernel parameter 'kaslr_boot_mem=nn@ss' to let users to >> > > specify the memory regions where kernel can be allowed to randomize >> > > safely. >> > >> > Manual solutions like that are pretty suboptimal to users, aren't they? >> > >> > In what way does memory hotplug feature 'break'? Does it crash or >> > misbehave? Or >> > simply does it not allow the movement of the affected memory region, while >> > still >> > allowing the rest to be moved? >> >> AFAIT, if kernel is randomized into the movable memory region, the >> affected memory region can not be hot added/removed since it has kernel >> data. Surely, the system can still work, the unaffected part still can >> be moved. Still it will cause regression on memory hotplug. >> >> Mainly we parse SRAT table to get the ranges of memory provided by >> hot-added memory devices in initmem_init(), that's very late. During boot, >> we don't know it. Chao ever posted patches to grab SRAT at decompressing >> stage, the code is very complicated and not elegant, ACPI maintainer >> NACKed that.
Thanks for Ingo's suggestion and Baoquan's explaination. Yes, I did ever try to dig SRAT table in boot period in early RFC PATCH: https://lkml.org/lkml/2017/9/3/77 But the change is too huge so made this patchset to avoid this bug in a small change, which will not make the code looks messy. Thanks, Chao Fan > >So there's apparently a mis-design here: > > - KASLR needs to be done very early on during bootup: - it's not realistic to > expect KASLR to be done with a booted up kernel, because pointers to > various > KASLR-ed objects are already widely spread out in memory. > > - But for some unfathomable reason the memory hotplug attribute of memory > regions is not part of the regular memory map but part of late-init ACPI > data > structures. > >The right solution would be _not_ to fudge the KASLR location, but to provide >the >memory hotplug information to early code, preferably via the primary memory >map. >KASLR can then make use of it and avoid those regions, just like it avoids >other >memory regions already. > >In addition to that hardware makers (including virtualized hardware) should >also >fix their systems to provide memory hotplug information to early code. > >Thanks, > > Ingo > >