linux-kernel - Re: [PATCH 3/4] infiniband/mm: convert to the new put_user

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Date:   Mon, 1 Oct 2018 08:29:29 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     John Hubbard <jhubbard@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>,
        john.hubbard@...il.com, Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Doug Ledford <dledford@...hat.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Christian Benvenuti <benve@...co.com>
Subject: Re: [PATCH 3/4] infiniband/mm: convert to the new put_user_page()
 call

On Mon, Oct 01, 2018 at 05:50:13AM -0700, Christoph Hellwig wrote:
> 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.

I don't think the bio_vec is really a big improvement; it's just a (page,
offset, length) tuple.  Not to mention that due to the annoying divergence
between block and networking [1] this is actually a less useful interface.

I don't understand the dislike of the sg list.  Other than for special
cases which we should't be optimising for (ramfs, brd, loopback
filesystems), when we get a page to do I/O, we're going to want a dma
mapping for them.  It makes sense to already allocate space to store
the mapping at the outset.

[1] Can we ever admit that the bio_vec and the skb_frag_t are actually
the same thing?