Re: [PATCH v3 1/2] mm, hwpoison: Try to recover from copy-on write faults

[Miaohe Lin]

On 2022/10/22 4:01, Tony Luck wrote:
> If the kernel is copying a page as the result of a copy-on-write
> fault and runs into an uncorrectable error, Linux will crash because
> it does not have recovery code for this case where poison is consumed
> by the kernel.
> 
> It is easy to set up a test case. Just inject an error into a private
> page, fork(2), and have the child process write to the page.
> 
> I wrapped that neatly into a test at:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/aegl/ras-tools.git
> 
> just enable ACPI error injection and run:
> 
>   # ./einj_mem-uc -f copy-on-write
> 
> Add a new copy_user_highpage_mc() function that uses copy_mc_to_kernel()
> on architectures where that is available (currently x86 and powerpc).
> When an error is detected during the page copy, return VM_FAULT_HWPOISON
> to caller of wp_page_copy(). This propagates up the call stack. Both x86
> and powerpc have code in their fault handler to deal with this code by
> sending a SIGBUS to the application.
> 
> Note that this patch avoids a system crash and signals the process that
> triggered the copy-on-write action. It does not take any action for the
> memory error that is still in the shared page. To handle that a call to
> memory_failure() is needed. But this cannot be done from wp_page_copy()
> because it holds mmap_lock(). Perhaps the architecture fault handlers
> can deal with this loose end in a subsequent patch?
> 
> On Intel/x86 this loose end will often be handled automatically because
> the memory controller provides an additional notification of the h/w
> poison in memory, the handler for this will call memory_failure(). This
> isn't a 100% solution. If there are multiple errors, not all may be
> logged in this way.
> 
> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>

Thanks for your work, Tony.

> 
> ---
> Changes in V3:
>     Dan Williams
> 	Rename copy_user_highpage_mc() to copy_mc_user_highpage() for
> 	consistency with Linus' discussion on names of functions that
> 	check for machine check.
> 	Write complete functions for the have/have-not copy_mc_to_kernel
> 	cases (so grep shows there are two versions)
> 	Change __wp_page_copy_user() to return 0 for success, negative for fail
> 	[I picked -EAGAIN for both non-EHWPOISON cases]
> 
> Changes in V2:
>    Naoya Horiguchi:
> 	1) Use -EHWPOISON error code instead of minus one.
> 	2) Poison path needs also to deal with old_page
>    Tony Luck:
> 	Rewrote commit message
> 	Added some powerpc folks to Cc: list
> ---
>  include/linux/highmem.h | 24 ++++++++++++++++++++++++
>  mm/memory.c             | 30 ++++++++++++++++++++----------
>  2 files changed, 44 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index e9912da5441b..a32c64681f03 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -319,6 +319,30 @@ static inline void copy_user_highpage(struct page *to, struct page *from,
>  
>  #endif
>  
> +#ifdef copy_mc_to_kernel
> +static inline int copy_mc_user_highpage(struct page *to, struct page *from,
> +					unsigned long vaddr, struct vm_area_struct *vma)
> +{
> +	unsigned long ret;
> +	char *vfrom, *vto;
> +
> +	vfrom = kmap_local_page(from);
> +	vto = kmap_local_page(to);
> +	ret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);

In copy_user_highpage(), kmsan_unpoison_memory(page_address(to), PAGE_SIZE) is done after the copy when
__HAVE_ARCH_COPY_USER_HIGHPAGE isn't defined. Do we need to do something similar here? But I'm not familiar
with kmsan, so I can easy be wrong.

Anyway, this patch looks good to me. Thanks.

Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>

Thanks,
Miaohe Lin