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

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Message-ID: <20180929162117.GA31216@bombadil.infradead.org>
Date:   Sat, 29 Sep 2018 09:21:17 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     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 Fri, Sep 28, 2018 at 08:12:33PM -0700, John Hubbard wrote:
> >> +++ b/drivers/infiniband/core/umem.c
> >> @@ -60,7 +60,7 @@ static void __ib_umem_release(struct ib_device *dev, struct ib_umem *umem, int d
> >>  		page = sg_page(sg);
> >>  		if (!PageDirty(page) && umem->writable && dirty)
> >>  			set_page_dirty_lock(page);
> >> -		put_page(page);
> >> +		put_user_page(page);
> > 
> > Would it make sense to have a release/put_user_pages_dirtied to absorb
> > the set_page_dity pattern too? I notice in this patch there is some
> > variety here, I wonder what is the right way?
> > 
> > Also, I'm told this code here is a big performance bottleneck when the
> > number of pages becomes very long (think >> GB of memory), so having a
> > future path to use some kind of batching/threading sound great.
> 
> Yes. And you asked for this the first time, too. Consistent! :) Sorry for
> 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.