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

From: David Hildenbrand <david@redhat.com>
To: Al Viro <viro@zeniv.linux.org.uk>, David Howells <dhowells@redhat.com>
Cc: 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
Date: Fri, 27 Jan 2023 00:44:08 +0100	[thread overview]
Message-ID: <ba3adce1-ddea-98e0-fc3a-1cb660edae4c@redhat.com> (raw)
In-Reply-To: <Y9MAbYt6DIRFm954@ZenIV>

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