Re: [PATCH RFC 3/4] crash_dump: recover from a machine check in copy_oldmem_page()

[sashiko-bot@kernel.org]

> A capture kernel reads the memory of the crashed kernel, and that memory
> may well have uncorrectable errors. That is after all what the first
> kernel often died of. Every architecture that maps a page of the old
> kernel and copies it out does so with copy_to_iter(), and a machine
> check in that copy takes the capture kernel down.
> 
> On x86 a machine check in the kernel is only recovered from if the
> instruction that took it has a machine check safe exception table entry
> or copies from userspace (cf. error_context()). The copy of an old page
> is neither. Its source is kernel memory, whether it goes to the buffer
> of a read() or, since commit 4a22fd20379c ("vmcore: convert
> __read_vmcore to use an iov_iter"), into a kernel page for splice(),
> sendfile() and copy_file_range(). So a read of /proc/vmcore that runs
> into a poisoned page panics the capture kernel, and so do vmcore's own
> reads of the ELF headers and notes. Breno sees the result on a fleet:
> [ ... ]
> Fixes: 4ae362be5093 ("[PATCH] kdump: read previous kernel's memory")
> Fixes: 54c32021eb6f ("[PATCH] powerpc: Add arch-dependent copy_oldmem_page")
> Fixes: 4a22fd20379c ("vmcore: convert __read_vmcore to use an iov_iter")
> Cc: stable@vger.kernel.org
> Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261112-work-procfs-oldmem-v1-0-ecdcafd99265@kernel.org?part=3