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

From: John Hubbard <jhubbard@nvidia.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>,
	David Hildenbrand <david@redhat.com>,
	"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>
Subject: Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator
Date: Thu, 26 Jan 2023 14:49:17 -0800	[thread overview]
Message-ID: <bf3edf11-02d1-128c-ebc0-11bb38404ac9@nvidia.com> (raw)
In-Reply-To: <Y9MAbYt6DIRFm954@ZenIV>

On 1/26/23 14:36, Al Viro wrote:
...
>>> +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; thankfully, we don't need to - they are used only
> for fetches, so FOLL_GET is sufficient.  With your "we'll just pin them,
> source or destination" you won't be able to convert at least that
> call of iov_iter_get_pages2().  And there might be other similar cases;
> I won't swear there's more, but ISTR running into more than one of
> the "pin won't be OK here, but fortunately it's a data source" places.

Assuming that "page is a data source" means that we are writing out from
the page to a block device (so, a WRITE operation, which of course
actually *reads* from the page), then...

...one thing I'm worried about now is whether Jan's original problem
report [1] can be fixed, because that involves page writeback. And it
seems like we need to mark the pages involved as "maybe dma-pinned" via
FOLL_PIN pins, in order to solve it.

Or am I missing a key point (I hope)?

[1] https://lore.kernel.org/linux-mm/20180103100430.GE4911@quack2.suse.cz/T/#u

thanks,
-- 
John Hubbard
NVIDIA