RE: [PATCH v2] dax: enable dax fault handler to report VM_FAULT_HWPOISON

[Dan Williams]

Jane Chu wrote:
> When dax fault handler fails to provision the fault page due to
> hwpoison, it returns VM_FAULT_SIGBUS which lead to a sigbus delivered
> to userspace with .si_code BUS_ADRERR.  Channel dax backend driver's
> detection on hwpoison to the filesystem to provide the precise reason
> for the fault.

It's not yet clear to me by this description why this is an improvement
or will not cause other confusion. In this case the reason for the
SIGBUS is because the driver wants to prevent access to poison, not that
the CPU consumed poison. Can you clarify what is lost by *not* making
this change?

> 
> Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>
> ---
>  drivers/nvdimm/pmem.c | 2 +-
>  fs/dax.c              | 2 +-
>  include/linux/mm.h    | 2 ++
>  3 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index ceea55f621cc..46e094e56159 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -260,7 +260,7 @@ __weak long __pmem_direct_access(struct pmem_device *pmem, pgoff_t pgoff,
>  		long actual_nr;
>  
>  		if (mode != DAX_RECOVERY_WRITE)
> -			return -EIO;
> +			return -EHWPOISON;
>  
>  		/*
>  		 * Set the recovery stride is set to kernel page size because
> diff --git a/fs/dax.c b/fs/dax.c
> index 3e457a16c7d1..c93191cd4802 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1456,7 +1456,7 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
>  
>  		map_len = dax_direct_access(dax_dev, pgoff, PHYS_PFN(size),
>  				DAX_ACCESS, &kaddr, NULL);
> -		if (map_len == -EIO && iov_iter_rw(iter) == WRITE) {
> +		if (map_len == -EHWPOISON && iov_iter_rw(iter) == WRITE) {
>  			map_len = dax_direct_access(dax_dev, pgoff,
>  					PHYS_PFN(size), DAX_RECOVERY_WRITE,
>  					&kaddr, NULL);

This change results in EHWPOISON leaking to usersapce in the case of
read(2), that's not a return code that block I/O applications have ever
had to contend with before. Just as badblocks cause EIO to be returned,
so should poisoned cachelines for pmem.


> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1f79667824eb..e4c974587659 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3217,6 +3217,8 @@ static inline vm_fault_t vmf_error(int err)
>  {
>  	if (err == -ENOMEM)
>  		return VM_FAULT_OOM;
> +	else if (err == -EHWPOISON)
> +		return VM_FAULT_HWPOISON;
>  	return VM_FAULT_SIGBUS;
>  }
>  
> -- 
> 2.18.4
> 
>