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

[Jens Axboe]

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