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

[Jeff Layton]

On Mon, 2021-04-26 at 18:54 +0000, Al Viro wrote:
> 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?
> 

Not sure I understand the issue you see. If xas_retry returns true,
we'll restart, but we won't have called STEP yet for that entry. I
don't see how we'd retry there and have an issue with idempotency.

> > +		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;						\
> > +}