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:   Mon, 23 Jan 2023 13:38:31 +0000
From:   David Howells <dhowells@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     dhowells@...hat.com, 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

David Hildenbrand <david@...hat.com> wrote:

> 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 :)

Well, it simplifies things a bit.

I can make the new iov_iter_extract_pages() just do "pin" or "don't pin" and
do no ref-getting at all.  Things can be converted over to "unpin the pages or
doing nothing" as they're converted over to using iov_iter_extract_pages()
from iov_iter_get_pages*().

The block bio code then only needs a single bit of state: pinned or not
pinned.

For cifs RDMA, do I need to make it pass in FOLL_LONGTERM?  And does that need
a special cleanup?

sk_buff fragment handling could still be tricky.  I'm thinking that in that
code I'll need to store FOLL_GET/PIN in the bottom two bits of the frag page
pointer.  Sometimes it allocates a new page and attaches it (have ref);
sometimes it does zerocopy to/from a page (have pin) and sometimes it may be
pointing to a kernel buffer (don't pin or ref).

David