Messages in this thread | | | Date | Wed, 5 Sep 2018 16:15:31 +0800 | From | Baoquan He <> | Subject | Re: [PATCH 2/2] x86/mm/KASLR: Adjust the vmemmap size according to paging mode |
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On 09/04/18 at 11:13am, Kirill A. Shutemov wrote: > On Mon, Sep 03, 2018 at 10:52:13PM +0800, Baoquan He wrote: > > On 09/03/18 at 01:26pm, Kirill A. Shutemov wrote: > > > > > But there's corner case when struct page is unreasonably large and > > > > > vmemmap_size will be way to large. We probably have to report an error if > > > > > we cannot fit vmemmap properly into virtual memory layout. > > > > > > > > Hmm, sizeof(struct page) can't exceed one whole page surely, otherwise > > > > system bootup can't go over vmemmap initlization. Except of this, we may > > > > need think about the virtual memory layout which vmemmap can be allowed > > > > to occupy. > > > > > > > > If KASAN enabled, KASLR disabled, > > > > 4-level 1TB + 1TB hole (2TB) > > > > 5-level 512TB + 2034TB hole (2.5PB) > > > > > > > > If KASAN disabled, KASLR enabled, > > > > 4-level 1TB + 1TB hole + 16TB (18TB) > > > > 5-level 512TB + 2034TB hole + 8PB (10.5PB) > > > > > > > > So, as you can see, if add check in memory KASLR code, we should only > > > > consider KASLR enabled case. We possibly don't need to worry about > > > > 5-level case since the size 10.5PB is even bigger than the maximum > > > > physical RAM mapping size. For 4-level, 18TB align to multiples of 2, it > > > > will be 32 times of the current 1TB, then we usually assume 64 as the > > > > default value of sizeof(struct page), then 64*32 == 1024. So we can add ~~~64*32 = 2048 Sorry, I made mistake here. > > > > check like this, what do you think? Or any other idea? > > > > > > Looks reasonable to me. > > > > > > But I would have the BUILD_BUG_ON() in generic code. If you struct page is > > > more than 1/4 of PAGE_SIZE something is horribly broken. > > > > Just the 1/4 of PAGE_SIZE is based on analysis of KASLR case. If > > non-KASLR case, it may not be that value. > > Even if it technically possible to have struct page larger than > PAGE_SIZE/4, it's just insane. > > > Not sure if it's OK to put it in generic code, and haven't thought of a > > good place, maybe in setup_arch(), just at the beginning? > > I don't see an obvious place too. Maybe free_area_init_nodes()?
OK, you mean a more generic place, I only considered generic place in x86. The thing is not all ARCH-es set PAGE_SIZE as 4KB, e.g power and arm64 can have PAGE_SIZE of 64KB. For them, PAGE_SIZE/4, namely 16KB, is hardly reached. So my thought is either taking PAGE_SIZE/4 in x86 arch only, or using SZ_1K in free_area_init_nodes() as you suggested. What do you think?
Thanks Baoquan
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