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

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Date:   Thu, 26 Jan 2023 22:15:13 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Hildenbrand <david@...hat.com>
Cc:     David Howells <dhowells@...hat.com>,
        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 Mon, Jan 23, 2023 at 02:24:13PM +0100, David Hildenbrand wrote:
> 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 :)

Take a good look at iter_to_pipe().  It does *not* need to pin anything
(we have an ITER_SOURCE there); with this approach it will.  And it
will stuff those pinned references into a pipe, where they can sit
indefinitely.

IOW, I don't believe it's a usable approach.