On 26.01.23 23:36, Al Viro wrote:

On Thu, Jan 26, 2023 at 09:59:36PM +0000, Al Viro wrote:

quoted

On Thu, Jan 26, 2023 at 02:16:20PM +0000, David Howells wrote:

quoted

+/**
+ * iov_iter_extract_will_pin - Indicate how pages from the iterator will be retained
+ * @iter: The iterator
+ *
+ * Examine the iterator and indicate by returning true or false as to how, if
+ * at all, pages extracted from the iterator will be retained by the extraction
+ * function.
+ *
+ * %true indicates that the pages will have a pin placed in them that the
+ * caller must unpin.  This is must be done for DMA/async DIO to force fork()
+ * to forcibly copy a page for the child (the parent must retain the original
+ * page).
+ *
+ * %false indicates that no measures are taken and that it's up to the caller
+ * to retain the pages.
+ */
+static inline bool iov_iter_extract_will_pin(const struct iov_iter *iter)
+{
+	return user_backed_iter(iter);
+}
+

Wait a sec; why would we want a pin for pages we won't be modifying?
A reference - sure, but...

After having looked through the earlier iterations of the patchset -
sorry, but that won't fly for (at least) vmsplice().  There we can't
pin those suckers; 

We'll need a way to pass FOLL_LONGTERM to pin_user_pages_fast() to 
handle such long-term pinning as vmsplice() needs. But the release path 
(unpin) will be the same.

-- 
Thanks,

David / dhildenb