linux-ext4 - Re: [RFC PATCH v2 15/19] mm/gup: Introduce vaddr_pin

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Date:   Tue, 13 Aug 2019 08:47:06 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Theodore Ts'o <tytso@....edu>,
        John Hubbard <jhubbard@...dia.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>, linux-xfs@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-ext4@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v2 15/19] mm/gup: Introduce vaddr_pin_pages()

On Mon, Aug 12, 2019 at 02:48:55PM -0700, Ira Weiny wrote:
> On Mon, Aug 12, 2019 at 09:28:14AM -0300, Jason Gunthorpe wrote:
> > On Fri, Aug 09, 2019 at 03:58:29PM -0700, ira.weiny@...el.com wrote:
> > > From: Ira Weiny <ira.weiny@...el.com>
> > > 
> > > The addition of FOLL_LONGTERM has taken on additional meaning for CMA
> > > pages.
> > > 
> > > In addition subsystems such as RDMA require new information to be passed
> > > to the GUP interface to track file owning information.  As such a simple
> > > FOLL_LONGTERM flag is no longer sufficient for these users to pin pages.
> > > 
> > > Introduce a new GUP like call which takes the newly introduced vaddr_pin
> > > information.  Failure to pass the vaddr_pin object back to a vaddr_put*
> > > call will result in a failure if pins were created on files during the
> > > pin operation.
> > 
> > Is this a 'vaddr' in the traditional sense, ie does it work with
> > something returned by valloc?
> 
> ...or malloc in user space, yes.  I think the idea is that it is a user virtual
> address.

valloc is a kernel call

> So I'm open to suggestions.  Jan gave me this one, so I figured it was safer to
> suggest it...

Should have the word user in it, imho

> > I also wish GUP like functions took in a 'void __user *' instead of
> > the unsigned long to make this clear :\
> 
> Not a bad idea.  But I only see a couple of call sites who actually use a 'void
> __user *' to pass into GUP...  :-/
> 
> For RDMA the address is _never_ a 'void __user *' AFAICS.

That is actually a bug, converting from u64 to a 'user VA' needs to go
through u64_to_user_ptr().

Jason