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Date: Fri, 14 Jun 2024 19:32:45 -0700
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Subject: Re: [PATCH v2 1/4] kernfs: remove page_mkwrite() from
 vm_operations_struct
To: Martin Oliveira <martin.oliveira@eideticom.com>,
	<linux-rdma@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<linux-mm@kvack.org>
CC: Jason Gunthorpe <jgg@ziepe.ca>, Leon Romanovsky <leon@kernel.org>, "Greg
 Kroah-Hartman" <gregkh@linuxfoundation.org>, Tejun Heo <tj@kernel.org>,
	"Andrew Morton" <akpm@linux-foundation.org>, Logan Gunthorpe
	<logang@deltatee.com>, Mike Marciniszyn <mike.marciniszyn@intel.com>, Shiraz
 Saleem <shiraz.saleem@intel.com>, Michael Guralnik <michaelgur@nvidia.com>,
	"Artemy Kovalyov" <artemyko@nvidia.com>
References: <20240611182732.360317-1-martin.oliveira@eideticom.com>
 <20240611182732.360317-2-martin.oliveira@eideticom.com>
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From: John Hubbard <jhubbard@nvidia.com>
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On 6/11/24 11:27 AM, Martin Oliveira wrote:
> The .page_mkwrite operator of kernfs just calls file_update_time().
> This is the same behaviour that the fault code does if .page_mkwrite is
> not set.
> 
> Furthermore, having the page_mkwrite() operator causes
> writable_file_mapping_allowed() to fail due to
> vma_needs_dirty_tracking() on the gup flow, which is a pre-requisite for
> enabling P2PDMA over RDMA.
> 
> There are no users of .page_mkwrite and no known valid use cases, so
> just remove the .page_mkwrite from kernfs_ops and return -EINVAL if an
> mmap() implementation sets .page_mkwrite.

Hi Martin and Logan!

First of all, I admire this approach to solving one of the gup+filesystem
interaction problems, by coming in from the other direction. Neat. :)


> 
> Co-developed-by: Logan Gunthorpe <logang@deltatee.com>
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> Signed-off-by: Martin Oliveira <martin.oliveira@eideticom.com>
> ---
>   fs/kernfs/file.c | 26 +++-----------------------
>   1 file changed, 3 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> index 8502ef68459b9..a198cb0718772 100644
> --- a/fs/kernfs/file.c
> +++ b/fs/kernfs/file.c
> @@ -386,28 +386,6 @@ static vm_fault_t kernfs_vma_fault(struct vm_fault *vmf)
>   	return ret;
>   }
>   
> -static vm_fault_t kernfs_vma_page_mkwrite(struct vm_fault *vmf)
> -{
> -	struct file *file = vmf->vma->vm_file;
> -	struct kernfs_open_file *of = kernfs_of(file);
> -	vm_fault_t ret;
> -
> -	if (!of->vm_ops)
> -		return VM_FAULT_SIGBUS;
> -
> -	if (!kernfs_get_active(of->kn))
> -		return VM_FAULT_SIGBUS;
> -
> -	ret = 0;
> -	if (of->vm_ops->page_mkwrite)
> -		ret = of->vm_ops->page_mkwrite(vmf);
> -	else
> -		file_update_time(file);
> -
> -	kernfs_put_active(of->kn);
> -	return ret;
> -}
> -
>   static int kernfs_vma_access(struct vm_area_struct *vma, unsigned long addr,
>   			     void *buf, int len, int write)
>   {
> @@ -432,7 +410,6 @@ static int kernfs_vma_access(struct vm_area_struct *vma, unsigned long addr,
>   static const struct vm_operations_struct kernfs_vm_ops = {
>   	.open		= kernfs_vma_open,
>   	.fault		= kernfs_vma_fault,
> -	.page_mkwrite	= kernfs_vma_page_mkwrite,
>   	.access		= kernfs_vma_access,
>   };
>   
> @@ -482,6 +459,9 @@ static int kernfs_fop_mmap(struct file *file, struct vm_area_struct *vma)
>   	if (vma->vm_ops && vma->vm_ops->close)
>   		goto out_put;
>   
> +	if (vma->vm_ops->page_mkwrite)

As the kernel test bot results imply, you probably want to do it like this:

    	if (vma->vm_ops && vma->vm_ops->page_mkwrite)


> +		goto out_put;
> +
>   	rc = 0;
>   	if (!of->mmapped) {
>   		of->mmapped = true;

thanks,
-- 
John Hubbard
NVIDIA