On 22 Apr 2023 14:37:05 +0100 Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> @@ -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);
> +}
