Re: [Crash-utility] [PATCH 2/2] arm64: fix the "pud page" crash for live system

Hi Kazu, On Tue, Mar 22, 2022 at 07:29:41AM +0000, HAGIO KAZUHITO(萩尾 一仁) wrote: > -----Original Message----- > > 1.) When I tested live system with "crash vmlinux /proc/kcore" in kernel v5.7, > > I met the following crash issue: > > ........................................ > > crash: seek error: kernel virtual address: ffff75e9fffff000 type: "pud page" > > ........................................ > > > > 2.) The root cause is the PTOV does not work correctly for some kernel, > > and then arm64_vtop_4level_4k() does not work correctly too. > > > > Why PTOV does not work? > > > > The PHYS_OFFSET is just wrapper of memstart_addr. > > ............................... > > #define PHYS_OFFSET ({ VM_BUG_ON(memstart_addr & 1); memstart_addr; }) > > ............................... > > > > Because memstart_addr is changed after physvirt_offset is initialized. > > so the NUMBER(PHYS_OFFSET) does not return the correct value. > > Hmm, it looks like arm64_PTOV() uses the physvirt_offset if available > (5.4 <= kernel < 5.10), I'm still not sure why it does not work correctly. > Would you please explain the issue in more detail? In arm64_calc_physvirt_offset(void), we try to read out the physvirt_offset from the live system, and we call read_proc_kcore(). In the read_proc_kcore: ................................................ if (paddr == KCORE_USE_VADDR) kvaddr = addr; else kvaddr = PTOV((ulong)paddr); ................................................ We need the PTOV (arm64_PTOV()) to work correctly. Unfortunately, arm64_PTOV() uses the ms->phys_offset again.. ................................................ ulong arm64_PTOV(ulong paddr) { struct machine_specific *ms = machdep->machspec; /* * Either older kernel before kernel has 'physvirt_offset' or newer * kernel which removes 'physvirt_offset' has the same formula: * #define __phys_to_virt(x) ((unsigned long)((x) - PHYS_OFFSET) | PAGE_OFFSET) */ if (!(machdep->flags & HAS_PHYSVIRT_OFFSET)) return (paddr - ms->phys_offset) | PAGE_OFFSET; else return paddr - ms->physvirt_offset; } ................................................ So the physvirt_offset depends on the NUMBER(PHYS_OFFSET).