Messages in this thread | | | Date | Mon, 28 Jul 2014 19:41:07 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 2/2] ARM: LPAE: reduce damage caused by idmap to virtual memory layout |
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On Mon, Jul 28, 2014 at 07:25:14PM +0100, Konstantin Khlebnikov wrote: > On Mon, Jul 28, 2014 at 10:14 PM, Will Deacon <will.deacon@arm.com> wrote: > > On Tue, Jul 22, 2014 at 04:36:35PM +0100, Konstantin Khlebnikov wrote: > >> idmap layout combines both phisical and virtual addresses. > >> Everything works fine if ram physically lays below PAGE_OFFSET. > >> Otherwise idmap starts punching huge holes in virtual memory layout. > >> It maps ram by 2MiB sections, but when it allocates new pmd page it > >> cuts 1GiB at once. > >> > >> This patch makes a copy of all affected pmds from init_mm. > >> Only few (usually one) 2MiB sections will be lost. > >> This is not eliminates problem but makes it 512 times less likely. > > > > I'm struggling to understand your commit message, but making a problem `512 > > times less likely' does sound like a bit of a hack to me. Can't we fix this > > properly instead? > > Yep, my comment sucks. > > Usually idmap looks like this: > > |0x00000000 -- <chunk of physical memory in identical mapping > --- | > TASK_SIZE -- <kernel space vm layoyt> --- 0xFFFFFFFF | > > But when that physical memory chunk starts from 0xE8000000 or even > 0xF2000000 evenything becomes very complicated.
Why? As long as we don't clobber the kernel text (which would require PHYS_OFFSET to be at a really weird alignment and very close to PAGE_OFFSET), then you should be alright. Sure, you'll lose things like your stack and the vmalloc area etc, but you're running in the idmap, so don't use those things.
soft_restart is an example of code that deals with these issues. Which code is causing you problems?
Will
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