linux-kernel - Re: [PATCH v4 2/3] mm: introduce put_user

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Message-Id: <20181009162012.c662ef0b041993557e150035@linux-foundation.org>
Date:   Tue, 9 Oct 2018 16:20:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jan Kara <jack@...e.cz>
Cc:     john.hubbard@...il.com, Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jerome Glisse <jglisse@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder
 versions

On Tue, 9 Oct 2018 10:30:25 +0200 Jan Kara <jack@...e.cz> wrote:

> > Also, maintainability.  What happens if someone now uses put_page() by
> > mistake?  Kernel fails in some mysterious fashion?  How can we prevent
> > this from occurring as code evolves?  Is there a cheap way of detecting
> > this bug at runtime?
> 
> The same will happen as with any other reference counting bug - the special
> user reference will leak. It will be pretty hard to debug I agree. I was
> thinking about whether we could provide some type safety against such bugs
> such as get_user_pages() not returning struct page pointers but rather some
> other special type but it would result in a big amount of additional churn
> as we'd have to propagate this different type e.g. through the IO path so
> that IO completion routines could properly call put_user_pages(). So I'm
> not sure it's really worth it.

I'm not really understanding.  Patch 3/3 changes just one infiniband
driver to use put_user_page().  But the changelogs here imply (to me)
that every user of get_user_pages() needs to be converted to
s/put_page/put_user_page/.

Methinks a bit more explanation is needed in these changelogs?