Re: [RFC PATCH v2 1/5] mm: Enable fault around for shared file page fault

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Feb 01, 2023 at 04:17:33PM +0800, Yin Fengwei wrote:
> The shared fault handler can also benefit from fault-around.  While it
> is uncommon to write to MAP_SHARED files, applications that do will see
> a huge benefit with will-it-scale:page_fault3 (shared file write fault)
> improving by 375%.
> 
> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/memory.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 7a04a1130ec1..51c04bb60724 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4546,6 +4546,17 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf)
>  	struct vm_area_struct *vma = vmf->vma;
>  	vm_fault_t ret, tmp;
>  
> +	/*
> +	 * Let's call ->map_pages() first and use ->fault() as fallback
> +	 * if page by the offset is not ready to be mapped (cold cache or
> +	 * something).
> +	 */
> +	if (should_fault_around(vmf)) {
> +		ret = do_fault_around(vmf);
> +		if (ret)
> +			return ret;
> +	}
> +

I believe it bypasses ->page_mkwrite() completely, no?

So you get a writable PTEs without notifying the filesystem. Smells like a
data loss.

>  	ret = __do_fault(vmf);
>  	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
>  		return ret;
> -- 
> 2.30.2
> 
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov