On Thu, 28 Jan 2016 21:12:54 +0800 Xunlei Pang <xpang at redhat.com> wrote: > On 2016/01/28 at 20:44, Michael Holzheu wrote: > > On Thu, 28 Jan 2016 19:56:56 +0800 > > Xunlei Pang <xpang at redhat.com> wrote: > > > >> On 2016/01/28 at 18:32, Michael Holzheu wrote: > >>> On Wed, 27 Jan 2016 11:15:46 -0800 > >>> Andrew Morton <akpm at linux-foundation.org> wrote: > >>> > >>>> On Wed, 27 Jan 2016 14:48:31 +0300 Dmitry Safonov <dsafonov at virtuozzo.com> wrote: > >>>> > >>>>> For allocation of kimage failure or kexec_prepare or load segments > >>>>> errors there is no need to keep crashkernel memory mapped. > >>>>> It will affect only s390 as map/unmap hook defined only for it. > >>>>> As on unmap s390 also changes os_info structure let's check return code > >>>>> and add info only on success. > >>>>> > >>>> This conflicts (both mechanically and somewhat conceptually) with > >>>> Xunlei Pang's "kexec: Introduce a protection mechanism for the > >>>> crashkernel reserved memory" and "kexec: provide > >>>> arch_kexec_protect(unprotect)_crashkres()". > >>>> > >>>> http://ozlabs.org/~akpm/mmots/broken-out/kexec-introduce-a-protection-mechanism-for-the-crashkernel-reserved-memory.patch > >>>> http://ozlabs.org/~akpm/mmots/broken-out/kexec-introduce-a-protection-mechanism-for-the-crashkernel-reserved-memory-v4.patch > >>>> > >>>> and > >>>> > >>>> http://ozlabs.org/~akpm/mmots/broken-out/kexec-provide-arch_kexec_protectunprotect_crashkres.patch > >>>> http://ozlabs.org/~akpm/mmots/broken-out/kexec-provide-arch_kexec_protectunprotect_crashkres-v4.patch > >>> Hmm, It looks to me that arch_kexec_(un)protect_crashkres() has exactly > >>> the same semantics as crash_(un)map_reserved_pages(). > >>> > >>> On s390 we don't have the crashkernel memory mapped and therefore need > >>> crash_map_reserved_pages() before loading something into crashkernel > >>> memory. > >> I don't know s390, just curious, if s390 doesn't have crash kernel memory mapped, > >> what's the purpose of the commit(558df7209e) for s390 as the reserved crash memory > >> with no kernel mapping already means the protection is on? > > When we reserve crashkernel memory on s390 ("crashkernel=" kernel parameter), > > we create a memory hole without page tables. > > > > Commit (558df7209e) was necessary to load a kernel/ramdisk into > > the memory hole with the kexec() system call. > > > > We create a temporary mapping with crash_map_reserved_pages(), then > > copy the kernel/ramdisk and finally remove the mapping again > > via crash_unmap_reserved_pages(). > > Thanks for the explanation. > So, on s390 the physical memory address has the same value as its kernel virtual address, > and kmap() actually returns the value of the physical address of the page, right? Correct. On s390 kmap() always return the physical address of the page. We have an 1:1 mapping for all the physical memory. For this area virtual=real. In addition to that we have the vmalloc area above the 1:1 mapping where some of the memory is mapped a second time. Michael
