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

[David Hildenbrand]

On 27.01.23 13:30, Jan Kara wrote:
> On Fri 27-01-23 02:02:31, Al Viro wrote:
> > On Fri, Jan 27, 2023 at 12:44:08AM +0100, David Hildenbrand wrote:
> > > 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.
> >
> > Umm...  Are you saying that if the source area contains DAX mmaps, vmsplice()
> > from it will fail?
>
> Yes, that's the plan. Because as you wrote elsewhere, it is otherwise too easy
> to lock up operations such as truncate(2) on DAX filesystems.

Right, it's then the same behavior as we already have for other 
FOLL_LONGTERM users, such as RDMA or io_uring.

... if we're afraid of breaking existing setups we could add some kind 
of fallback to copy to a buffer like ordinary pipe writes.

--
Thanks,

David / dhildenb