Re: [PATCH v7 01/31] iov_iter: Add ITER_XARRAY

Al Viro
2021-04-26T18:54:53Z

On Fri, Apr 23, 2021 at 02:28:01PM +0100, David Howells wrote:
> -#define iterate_all_kinds(i, n, v, I, B, K) { \
> +#define iterate_xarray(i, n, __v, skip, STEP) { \
> + struct page *head = NULL; \
> + size_t wanted = n, seg, offset; \
> + loff_t start = i->xarray_start + skip; \
> + pgoff_t index = start >> PAGE_SHIFT; \
> + int j; \
> + \
> + XA_STATE(xas, i->xarray, index); \
> + \
> + rcu_read_lock(); \
> + xas_for_each(&xas, head, ULONG_MAX) { \
> + if (xas_retry(&xas, head)) \
> + continue; \

OK, now I'm really confused; what's to guarantee that restart will not have
you hit the same entry more than once? STEP might be e.g.

memcpy_to_page(v.bv_page, v.bv_offset,
(from += v.bv_len) - v.bv_len, v.bv_len)

which is clearly not idempotent - from gets incremented, after all.
What am I missing here?

> + if (WARN_ON(xa_is_value(head))) \
> + break; \
> + if (WARN_ON(PageHuge(head))) \
> + break; \
> + for (j = (head->index < index) ? index - head->index : 0; \
> + j < thp_nr_pages(head); j++) { \
> + __v.bv_page = head + j; \
> + offset = (i->xarray_start + skip) & ~PAGE_MASK; \
> + seg = PAGE_SIZE - offset; \
> + __v.bv_offset = offset; \
> + __v.bv_len = min(n, seg); \
> + (void)(STEP); \
> + n -= __v.bv_len; \
> + skip += __v.bv_len; \
> + if (n == 0) \
> + break; \
> + } \
> + if (n == 0) \
> + break; \
> + } \
> + rcu_read_unlock(); \
> + n = wanted - n; \
> +}