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Date:   Mon, 24 Apr 2023 15:02:47 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by
 default
Message-ID: <20230424120247.k7cjmncmov32yv5r@box.shutemov.name>
References: <f86dc089b460c80805e321747b0898fd1efe93d7.1682168199.git.lstoakes@gmail.com>
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On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote:
> @@ -959,16 +959,46 @@ static int faultin_page(struct vm_area_struct *vma,
>  	return 0;
>  }
>  
> +/*
> + * Writing to file-backed mappings using GUP is a fundamentally broken operation
> + * as kernel write access to GUP mappings may not adhere to the semantics
> + * expected by a file system.
> + *
> + * In most instances we disallow this broken behaviour, however there are some
> + * exceptions to this enforced here.
> + */
> +static inline bool can_write_file_mapping(struct vm_area_struct *vma,
> +					  unsigned long gup_flags)
> +{
> +	struct file *file = vma->vm_file;
> +
> +	/* If we aren't pinning then no problematic write can occur. */
> +	if (!(gup_flags & (FOLL_GET | FOLL_PIN)))
> +		return true;
> +
> +	/* Special mappings should pose no problem. */
> +	if (!file)
> +		return true;
> +
> +	/* Has the caller explicitly indicated this case is acceptable? */
> +	if (gup_flags & FOLL_ALLOW_BROKEN_FILE_MAPPING)
> +		return true;
> +
> +	/* shmem and hugetlb mappings do not have problematic semantics. */
> +	return vma_is_shmem(vma) || is_file_hugepages(file);

Can this be generalized to any fs that doesn't have vm_ops->page_mkwrite()?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov