linux-kernel - Re: [PATCH v4 7/7] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

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Date:   Sun, 8 Jan 2023 20:54:12 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 7/7] iov_iter, block: Make bio structs pin pages rather
 than ref'ing if appropriate

On 1/6/23 5:34 PM, David Howells wrote:
> Convert the block layer's bio code to use iov_iter_extract_pages() instead
> of iov_iter_get_pages().  This will pin pages or leave them unaltered
> rather than getting a ref on them as appropriate to the source iterator.
> 
> A field, bi_cleanup_mode, is added to the bio struct that gets set by
> iov_iter_extract_pages() with FOLL_* flags indicating what cleanup is
> necessary.  FOLL_GET -> put_page(), FOLL_PIN -> unpin_user_page().  Other
> flags could also be used in future.
> 
> Newly allocated bio structs have bi_cleanup_mode set to FOLL_GET to
> indicate that attached pages are ref'd by default.  Cloning sets it to 0.
> __bio_iov_iter_get_pages() overrides it to what iov_iter_extract_pages()
> indicates.

What's the motivation for this change? It's growing struct bio, which we
can have a lot of in the system. I read the cover letter too and I can
tell what the change does, but there's no justification really for the
change.

So unless there's a good reason to do this, then that's a NAK in terms
of just the addition to struct bio alone.

-- 
Jens Axboe