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

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Date:   Fri, 27 Jan 2023 02:02:31 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Hildenbrand <david@...hat.com>
Cc:     David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list
 from an iterator

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?