Re: [PATCH 10/20] mm: Move handling of COW faults into DAX code

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Tue, Sep 27, 2016 at 06:08:14PM +0200, Jan Kara wrote:
> Move final handling of COW faults from generic code into DAX fault
> handler. That way generic code doesn't have to be aware of peculiarities
> of DAX locking so remove that knowledge.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/dax.c            | 22 ++++++++++++++++------
>  include/linux/dax.h |  7 -------
>  include/linux/mm.h  |  9 +--------
>  mm/memory.c         | 14 ++++----------
>  4 files changed, 21 insertions(+), 31 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 0dc251ca77b8..b1c503930d1d 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -876,10 +876,15 @@ int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
>  			goto unlock_entry;
>  		if (!radix_tree_exceptional_entry(entry)) {
>  			vmf->page = entry;
> -			return VM_FAULT_LOCKED;
> +			if (unlikely(PageHWPoison(entry))) {
> +				put_locked_mapping_entry(mapping, vmf->pgoff,
> +							 entry);
> +				return VM_FAULT_HWPOISON;
> +			}
>  		}
> -		vmf->entry = entry;
> -		return VM_FAULT_DAX_LOCKED;
> +		error = finish_fault(vmf);
> +		put_locked_mapping_entry(mapping, vmf->pgoff, entry);
> +		return error ? error : VM_FAULT_DONE_COW;
>  	}
>  
>  	if (!buffer_mapped(&bh)) {
> @@ -1430,10 +1435,15 @@ int iomap_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
>  			goto unlock_entry;
>  		if (!radix_tree_exceptional_entry(entry)) {
>  			vmf->page = entry;

In __do_fault() we explicitly clear vmf->page in the case where PageHWPoison()
is set.  I think we can get the same behavior here by moving the call that
sets vmf->page after the PageHWPoison() check.

> -			return VM_FAULT_LOCKED;
> +			if (unlikely(PageHWPoison(entry))) {
> +				put_locked_mapping_entry(mapping, vmf->pgoff,
> +							 entry);
> +				return VM_FAULT_HWPOISON;
> +			}
>  		}
> -		vmf->entry = entry;
> -		return VM_FAULT_DAX_LOCKED;

I think we're missing a call to 

	__SetPageUptodate(new_page);

before finish_fault()?  This call currently lives in do_cow_fault(), and
is part of the path that we don't skip as part of the VM_FAULT_DAX_LOCKED
logic.

Both of these comments apply equally to the iomap_dax_fault() code.

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