Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default

[Hillf Danton <hdanton@sina.com>]

On 22 Apr 2023 14:37:05 +0100 Lorenzo Stoakes <lstoakes@gmail.com>
> @@ -110,7 +110,8 @@ int qib_get_user_pages(unsigned long start_page, size_t num_pages,
>  	for (got = 0; got < num_pages; got += ret) {
>  		ret = pin_user_pages(start_page + got * PAGE_SIZE,
>  				     num_pages - got,
> -				     FOLL_LONGTERM | FOLL_WRITE,
> +				     FOLL_LONGTERM | FOLL_WRITE |
> +				     FOLL_ALLOW_BROKEN_FILE_MAPPING,
>  				     p + got, NULL);
>  		if (ret < 0) {
>  			mmap_read_unlock(current->mm);

[...]

> +/*
> + * 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;

Does the caller mean that PUP for DMA is no longer breaking writeback?
> +
> +	/* shmem and hugetlb mappings do not have problematic semantics. */
> +	return vma_is_shmem(vma) || is_file_hugepages(file);
> +}