linux-kernel - Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator

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Message-ID: <c742e47b-dcc0-1fef-dc8c-3bf85d26b046@redhat.com>
Date:   Mon, 23 Jan 2023 14:24:13 +0100
From:   David Hildenbrand <david@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        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>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>, linux-mm@...ck.org
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list
 from an iterator

On 23.01.23 14:19, David Howells wrote:
> David Hildenbrand <david@...hat.com> wrote:
> 
>> Switching from FOLL_GET to FOLL_PIN was in the works by John H. Not sure what
>> the status is. Interestingly, Documentation/core-api/pin_user_pages.rst
>> already documents that "CASE 1: Direct IO (DIO)" uses FOLL_PIN ... which does,
>> unfortunately, no reflect reality yet.
> 
> Yeah - I just came across that.
> 
> Should iov_iter.c then switch entirely to using pin_user_pages(), rather than
> get_user_pages()?  In which case my patches only need keep track of
> pinned/not-pinned and never "got".

That would be the ideal case: whenever intending to access page content, 
use FOLL_PIN instead of FOLL_GET.

The issue that John was trying to sort out was that there are plenty of 
callsites that do a simple put_page() instead of calling 
unpin_user_page(). IIRC, handling that correctly in existing code -- 
what was pinned must be released via unpin_user_page() -- was the 
biggest workitem.

Not sure how that relates to your work here (that's why I was asking): 
if you could avoid FOLL_GET, that would be great :)

-- 
Thanks,

David / dhildenb