Re: [RFC PATCH] mm, hwpoison: Recover from copy-on-write machine checks

[HORIGUCHI NAOYA(堀口 直也)]

On Mon, Oct 17, 2022 at 04:42:03PM -0700, 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
> 
> [Note this test needs some better reporting for the case where this
> patch has been applied and the system does NOT crash]
> 
> Patch below works ... but there are probably many places where it could
> fit better into the general "mm" way of doing things. E.g. using the
> copy_mc_to_kernel() function does what I need here, but the name doesn't
> seem like it is quite right.

As for the name "copy_user_highpage_mc", it simply sounds like an mcsafe
variant of copy_user_highpage, so I have no objection.

Recently similar routine is suggested in
https://lore.kernel.org/linux-mm/20221010160142.1087120-2-jiaqiyan@xxxxxxxxxx/
, so maybe we need some coordination between related interfaces.

> 
> Basic idea is very simple ... if the kernel gets a machine check copying
> the page, just free up the new page that was going to be the target of
> the copy and return VM_FAULT_HWPOISON to the calling stack.
> 
> Slightly-signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>

I'm basically supportive to what this patch intends to do.

> ---
>  include/linux/highmem.h | 19 +++++++++++++++++++
>  mm/memory.c             | 28 ++++++++++++++++++++--------
>  2 files changed, 39 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index e9912da5441b..5967541fbf0e 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -319,6 +319,25 @@ static inline void copy_user_highpage(struct page *to, struct page *from,
>  
>  #endif
>  
> +static inline int copy_user_highpage_mc(struct page *to, struct page *from,
> +					unsigned long vaddr, struct vm_area_struct *vma)
> +{
> +	unsigned long ret = 0;
> +#ifdef copy_mc_to_kernel
> +	char *vfrom, *vto;
> +
> +	vfrom = kmap_local_page(from);
> +	vto = kmap_local_page(to);
> +	ret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);
> +	kunmap_local(vto);
> +	kunmap_local(vfrom);
> +#else
> +	copy_user_highpage(to, from, vaddr, vma);
> +#endif
> +
> +	return ret;
> +}
> +
>  #ifndef __HAVE_ARCH_COPY_HIGHPAGE
>  
>  static inline void copy_highpage(struct page *to, struct page *from)
> diff --git a/mm/memory.c b/mm/memory.c
> index f88c351aecd4..b5e22bf4c10a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2848,8 +2848,14 @@ static inline int pte_unmap_same(struct vm_fault *vmf)
>  	return same;
>  }
>  
> -static inline bool __wp_page_copy_user(struct page *dst, struct page *src,
> -				       struct vm_fault *vmf)
> +/*
> + * Return:
> + *	-1 = copy failed due to poison in source page

Simply calling "poison" might cause confusion with page poisoning feature,
so "hwpoison" might be better.  But I know that "poison" is commonly used
under arch/x86, and it's not clear to me what to do with this terminology.
Maybe using -EHWPOISON instead of -1 might be helpful to the distinction.

> + *	0 = copied failed (some other reason)
> + *	1 = copied succeeded
> + */
> +static inline int __wp_page_copy_user(struct page *dst, struct page *src,
> +				      struct vm_fault *vmf)
>  {
>  	bool ret;
>  	void *kaddr;
...
> @@ -3121,7 +3129,11 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>  		if (!new_page)
>  			goto oom;
>  
> -		if (!__wp_page_copy_user(new_page, old_page, vmf)) {
> +		ret = __wp_page_copy_user(new_page, old_page, vmf);
> +		if (ret == -1) {
> +			put_page(new_page);

Maybe I miss something, but don't you have to care about refcount of
old_page in this branch (as done in "ret == 0" branch)?

Thanks,
Naoya Horiguchi

> +			return VM_FAULT_HWPOISON;
> +		} else if (ret == 0) {
>  			/*
>  			 * COW failed, if the fault was solved by other,
>  			 * it's fine. If not, userspace would re-fault on
> -- 
> 2.37.3