Re: [PATCH v20 29/32] block: Replace BIO_NO_PAGE_REF with BIO_PAGE_REFFED with inverted logic

[David Howells]

Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote:

> > Replace BIO_NO_PAGE_REF with a BIO_PAGE_REFFED flag that has the inverted
> > meaning is only set when a page reference has been acquired that needs to
> > be released by bio_release_pages().
> 
> What was the motivation for this patch?

We need to move to using FOLL_PIN for buffers derived from direct I/O to avoid
the fork vs async-DIO race.  Further, we shouldn't be taking a ref or a pin on
pages derived from internal kernel iterators such as KVEC or BVEC as the page
refcount might not be a valid way to control the lifetime of the data/buffers
in those pages (slab, for instance).  Rather, for internal kernel I/O, we need
to rely on the caller to hold onto the memory until we tell them we've
finished.

So we flip the polarity of the page-is-ref'd flag and then add a
page-is-pinned flag.  The intention is to ultimately drop the page-is-ref'd
flag - but we still need to keep the page-is-pinned flag.  This makes it
easier to take a stepwise approach - and having both flags working the same
way makes the logic easier to follow.

See iov_iter_extract_pages() and iov_iter_extract_will_pin().

David