netdev - Re: [RESEND 4/7] mm/gup: Add FOLL_LONGTERM capability to GUP fast

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Date:   Mon, 25 Mar 2019 14:51:50 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        "David S. Miller" <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Rich Felker <dalias@...c.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>, linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mips@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Linux-sh <linux-sh@...r.kernel.org>, sparclinux@...r.kernel.org,
        linux-rdma@...r.kernel.org,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [RESEND 4/7] mm/gup: Add FOLL_LONGTERM capability to GUP fast

On Mon, Mar 25, 2019 at 02:23:15AM -0700, Ira Weiny wrote:
> > > Unfortunately holding the lock is required to support FOLL_LONGTERM (to check
> > > the VMAs) but we don't want to hold the lock to be optimal (specifically allow
> > > FAULT_FOLL_ALLOW_RETRY).  So I'm maintaining the optimization for *_fast users
> > > who do not specify FOLL_LONGTERM.
> > > 
> > > Another way to do this would have been to define __gup_longterm_unlocked with
> > > the above logic, but that seemed overkill at this point.
> > 
> > get_user_pages_unlocked() is an exported symbol, shouldn't it work
> > with the FOLL_LONGTERM flag?
> > 
> > I think it should even though we have no user..
> > 
> > Otherwise the GUP API just gets more confusing.
> 
> I agree WRT to the API.  But I think callers of get_user_pages_unlocked() are
> not going to get the behavior they want if they specify FOLL_LONGTERM.

Oh? Isn't the only thing FOLL_LONGTERM does is block the call on DAX?
Why does the locking mode matter to this test?

> What I could do is BUG_ON (or just WARN_ON) if unlocked is called with
> FOLL_LONGTERM similar to the code in get_user_pages_locked() which does not
> allow locked and vmas to be passed together:

The GUP call should fail if you are doing something like this. But I'd
rather not see confusing specialc cases in code without a clear
comment explaining why it has to be there.

Jason