# \[PATCH RFC 26/38\] powerpc/crash\_dump: recover from a machine check in copy\_oldmem\_page() - From: Christian Brauner \ - Date: 2026-11-02 14:27:46 - List: linux-fbdev - Page: https://ratatoskr.run/linux-fbdev/2026/11/17844962 - Thread: https://ratatoskr.run/linux-fsdevel/2026/11/17844895/t.md ```text A capture kernel reads the memory of the crashed kernel, and that memory may well have uncorrectable errors. On powerpc a machine check in the kernel is only recovered from if the instruction that took it has an exception table entry (cf. mce_common_process_ue()). copy_oldmem_page() copies with copy_to_iter(). For a read() of /proc/vmcore that is a user copy, all of whose loads have exception table entries, so the machine check turns into a short copy and read_from_oldmem() returns -EFAULT. But since commit 4a22fd20379c ("vmcore: convert __read_vmcore to use an iov_iter") /proc/vmcore can be spliced too, and splice(), sendfile() and copy_file_range() copy into kernel pages with memcpy(), which has no exception table entries. A machine check there panics the capture kernel. The same goes for the reads of the ELF headers and notes that vmcore does itself. Copy with _copy_mc_to_iter() instead. PPC64 selects ARCH_HAS_COPY_MC and there it uses copy_mc_generic() for kernel and user destinations alike, which has the exception table entries. Everywhere else it is _copy_to_iter(). Reported-by: Chris Mason Fixes: 4a22fd20379c ("vmcore: convert __read_vmcore to use an iov_iter") Cc: Signed-off-by: Christian Brauner (Amutable) --- arch/powerpc/kernel/crash_dump.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/powerpc/kernel/crash_dump.c b/arch/powerpc/kernel/crash_dump.c index 1543917bd3e1..ca7e019ae3e9 100644 --- a/arch/powerpc/kernel/crash_dump.c +++ b/arch/powerpc/kernel/crash_dump.c @@ -83,12 +83,12 @@ ssize_t copy_oldmem_page(struct iov_iter *iter, unsigned long pfn, if (memblock_is_region_memory(paddr, csize)) { vaddr = __va(paddr); - csize = copy_to_iter(vaddr + offset, csize, iter); + csize = _copy_mc_to_iter(vaddr + offset, csize, iter); } else { vaddr = ioremap_cache(paddr, PAGE_SIZE); if (!vaddr) return -ENOMEM; - csize = copy_to_iter(vaddr + offset, csize, iter); + csize = _copy_mc_to_iter(vaddr + offset, csize, iter); iounmap(vaddr); } -- 2.53.0 ```