Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator

[David Hildenbrand]

On 26.01.23 23:36, Al Viro wrote:
> On Thu, Jan 26, 2023 at 09:59:36PM +0000, Al Viro wrote:
> > On Thu, Jan 26, 2023 at 02:16:20PM +0000, David Howells wrote:
> >
> > > +/**
> > > + * 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