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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <c742e47b-dcc0-1fef-dc8c-3bf85d26b046@redhat.com>
References: <c742e47b-dcc0-1fef-dc8c-3bf85d26b046@redhat.com> <7bbcccc9-6ebf-ffab-7425-2a12f217ba15@redhat.com> <246ba813-698b-8696-7f4d-400034a3380b@redhat.com> <20230120175556.3556978-1-dhowells@redhat.com> <20230120175556.3556978-3-dhowells@redhat.com> <3814749.1674474663@warthog.procyon.org.uk> <3903251.1674479992@warthog.procyon.org.uk>
To:     David Hildenbrand <david@redhat.com>
Cc:     dhowells@redhat.com, Al Viro <viro@zeniv.linux.org.uk>,
        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>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        John Hubbard <jhubbard@nvidia.com>, linux-mm@kvack.org
Subject: 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
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David Hildenbrand <david@redhat.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 -- wha=
t was
> pinned must be released via unpin_user_page() -- was the biggest workite=
m.
> =

> Not sure how that relates to your work here (that's why I was asking): i=
f 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" a=
nd
do no ref-getting at all.  Things can be converted over to "unpin the page=
s 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 tha=
t
code I'll need to store FOLL_GET/PIN in the bottom two bits of the frag pa=
ge
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