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 00:44:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>
Cc:     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 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