Re: [PATCH 3/4] infiniband/mm: convert to the new put_user_page() call

[Christoph Hellwig <hch@infradead.org>]

On Sat, Sep 29, 2018 at 09:21:17AM -0700, Matthew Wilcox wrote:
> > being slow to pick it up. It looks like there are several patterns, and
> > we have to support both set_page_dirty() and set_page_dirty_lock(). So
> > the best combination looks to be adding a few variations of
> > release_user_pages*(), but leaving put_user_page() alone, because it's
> > the "do it yourself" basic one. Scatter-gather will be stuck with that.
> 
> I think our current interfaces are wrong.  We should really have a
> get_user_sg() / put_user_sg() function that will set up / destroy an
> SG list appropriate for that range of user memory.  This is almost
> orthogonal to the original intent here, so please don't see this as a
> "must do first" kind of argument that might derail the whole thing.

The SG list really is the wrong interface, as it mixes up information
about the pages/phys addr range and a potential dma mapping.  I think
the right interface is an array of bio_vecs.  In fact I've recently
been looking into a get_user_pages variant that does fill bio_vecs,
as it fundamentally is the right thing for doing I/O on large pages,
and will really help with direct I/O performance in that case.