Re: [PATCH v1 00/15] Keep track of GUPed pages in fs and block

[Dan Williams]

On Thu, Apr 18, 2019 at 3:42 AM Jan Kara <jack@xxxxxxx> wrote:
> > Except that this solution (biasing everyone in bio) would _more complex_
> > it is only conceptualy appealing. The changes are on the other hand much
> > deeper and much riskier but you decided to ignore that and focus on some-
> > thing i was just giving as an example.
>
> Yeah, after going and reading several places like fs/iomap.c, fs/mpage.c,
> drivers/md/dm-io.c I agree with you. The places that are not doing direct
> IO usually just don't hold any page reference that could be directly
> attributed to the bio (and they don't drop it when bio finishes). They
> rather use other means (like PageLocked, PageWriteback) to make sure the
> page stays alive so mandating gup-pin reference for all pages attached to a
> bio would require a lot of reworking of places that are not related to our
> problem and currently work just fine. So I withdraw my suggestion. Nice in
> theory, too much work in practice ;).

Is it though? We already have BIO_NO_PAGE_REF, so it seems it would be
a useful cleanup to have all locations that don't participate in page
references use that existing flag and then teach all other locations
to use gup-pinned pages.