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

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Message-ID: <20181009083025.GE11150@quack2.suse.cz>
Date:   Tue, 9 Oct 2018 10:30:25 +0200
From:   Jan Kara <jack@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>
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>,
        Jan Kara <jack@...e.cz>, 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 Mon 08-10-18 17:14:42, Andrew Morton wrote:
> On Mon,  8 Oct 2018 14:16:22 -0700 john.hubbard@...il.com wrote:
> > +		put_user_page(pages[index]);
> > +	}
> > +}
> > +
> > +static inline void put_user_pages(struct page **pages,
> > +				  unsigned long npages)
> > +{
> > +	unsigned long index;
> > +
> > +	for (index = 0; index < npages; index++)
> > +		put_user_page(pages[index]);
> > +}
> > +
> 
> Otherwise looks OK.  Ish.  But it would be nice if that comment were to
> explain *why* get_user_pages() pages must be released with
> put_user_page().

The reason is that eventually we want to track reference from GUP
separately but you're right that it would be good to have a comment about
that somewhere.

> 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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR