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

David Howells
2021-04-26T19:24:04Z

Al Viro <viro@zeniv.linux.org.uk> 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?

I really need to defer this question to Willy, but as I understand it,
xas_retry() only restarts the current iteration. Referring to the comment on
xas_reset():

* Resets the error or walk state of the @xas so future walks of the
* array will start from the root. Use this if you have dropped the
* xarray lock and want to reuse the xa_state.

I think that the walk returns to the bottom of the tree and whilst xarray
presents an interface that appears to be a contiguous array, it's actually a
tree internally - and 'root' is the root of the tree, not the head of the
array.

Basically, I think it throws away its cached iteration state - which might
have been modified - and rewalks the tree to get back to the same index.

David