2023-09-23 00:32:04

by David Howells

Subject: [PATCH v6 09/13] iov_iter: Add a kernel-type iterator-only iteration function

Add an iteration function that can only iterate over kernel internal-type
iterators (ie. BVEC, KVEC, XARRAY) and not user-backed iterators (ie. UBUF
and IOVEC). This allows for smaller iterators to be built when it is known
the caller won't have a user-backed iterator.

Signed-off-by: David Howells <[email protected]>
cc: Alexander Viro <[email protected]>
cc: Jens Axboe <[email protected]>
cc: Christoph Hellwig <[email protected]>
cc: Christian Brauner <[email protected]>
cc: Matthew Wilcox <[email protected]>
cc: Linus Torvalds <[email protected]>
cc: David Laight <[email protected]>
cc: [email protected]
cc: [email protected]
cc: [email protected]
---

Notes:
Changes
=======
ver #6)
- Document the priv2 arg of iterate_and_advance_kernel().

include/linux/iov_iter.h | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)

diff --git a/include/linux/iov_iter.h b/include/linux/iov_iter.h
index 270454a6703d..d8733dc22b54 100644
--- a/include/linux/iov_iter.h
+++ b/include/linux/iov_iter.h
@@ -271,4 +271,36 @@ size_t iterate_and_advance(struct iov_iter *iter, size_t len, void *priv,
return iterate_and_advance2(iter, len, priv, NULL, ustep, step);
}

+/**
+ * iterate_and_advance_kernel - Iterate over a kernel iterator
+ * @iter: The iterator to iterate over.
+ * @len: The amount to iterate over.
+ * @priv: Data for the step functions.
+ * @priv2: More data for the step functions.
+ * @step: Processing function; given kernel addresses.
+ *
+ * Like iterate_and_advance2(), but rejected UBUF and IOVEC iterators and does
+ * not take a user-step function.
+ */
+static __always_inline
+size_t iterate_and_advance_kernel(struct iov_iter *iter, size_t len, void *priv,
+ void *priv2, iov_step_f step)
+{
+ if (unlikely(iter->count < len))
+ len = iter->count;
+ if (unlikely(!len))
+ return 0;
+
+ if (iov_iter_is_bvec(iter))
+ return iterate_bvec(iter, len, priv, priv2, step);
+ if (iov_iter_is_kvec(iter))
+ return iterate_kvec(iter, len, priv, priv2, step);
+ if (iov_iter_is_xarray(iter))
+ return iterate_xarray(iter, len, priv, priv2, step);
+ if (iov_iter_is_discard(iter))
+ return iterate_discard(iter, len, priv, priv2, step);
+ WARN_ON_ONCE(1);
+ return 0;
+}
+
#endif /* _LINUX_IOV_ITER_H */

2023-09-23 02:21:16

by Linus Torvalds

Subject: Re: [PATCH v6 09/13] iov_iter: Add a kernel-type iterator-only iteration function

On Fri, 22 Sept 2023 at 05:03, David Howells <[email protected]> wrote:
>
> Add an iteration function that can only iterate over kernel internal-type
> iterators (ie. BVEC, KVEC, XARRAY) and not user-backed iterators (ie. UBUF
> and IOVEC). This allows for smaller iterators to be built when it is known
> the caller won't have a user-backed iterator.

This one is pretty ugly, and has no actual users.

Without even explaining why we'd care about this abomination, NAK.

If we actyually have some static knowledge of "this will only use
iterators X/Y/Z", then we should probably pass that in as a constant
bitmask to the thing, instead of this kind of "kernel only" special
case.

But even then, we'd want to have actual explicit use-cases, not a
hypothetical "if you have this situation here's this function".

Linus