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

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Message-ID: <cbca813e-cdaf-6938-570e-17dd26e3cd87@redhat.com>
Date:   Fri, 27 Jan 2023 13:34:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>
Cc:     David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, 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 27.01.23 13:30, Jan Kara wrote:
> 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.

Right, it's then the same behavior as we already have for other 
FOLL_LONGTERM users, such as RDMA or io_uring.

... if we're afraid of breaking existing setups we could add some kind 
of fallback to copy to a buffer like ordinary pipe writes.

-- 
Thanks,

David / dhildenb