Date:   Fri, 27 Jan 2023 13:30:30 +0100
From:   Jan Kara <jack@suse.cz>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     David Hildenbrand <david@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Christoph Hellwig <hch@lst.de>,
        John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list
 from an iterator
Message-ID: <20230127123030.qfmgkthuzlxadpkk@quack3>
References: <20230126141626.2809643-1-dhowells@redhat.com>
 <20230126141626.2809643-3-dhowells@redhat.com>
 <Y9L3yA+B1rrnrGK8@ZenIV>
 <Y9MAbYt6DIRFm954@ZenIV>
 <ba3adce1-ddea-98e0-fc3a-1cb660edae4c@redhat.com>
 <Y9Mwt1EMm8InCHvA@ZenIV>
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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.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR