Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 19 May 2020 08:32:51 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Laurent Dufour <ldufour@linux.ibm.com>
Cc:     Michel Lespinasse <walken@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Liam Howlett <Liam.Howlett@oracle.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        David Rientjes <rientjes@google.com>,
        Hugh Dickins <hughd@google.com>, Ying Han <yinghan@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH v5.5 10/10] mmap locking API: rename mmap_sem to mmap_lock
Message-ID: <20200519153251.GY16070@bombadil.infradead.org>
References: <20200422001422.232330-1-walken@google.com>
 <20200422001422.232330-11-walken@google.com>
 <20200422015829.GR5820@bombadil.infradead.org>
 <CANN689EnGsJXA8n6JvTryQfkCtARPvtZbkH+9Dd2a4X+fvqU9g@mail.gmail.com>
 <20200423015917.GA13910@bombadil.infradead.org>
 <20200424012612.GA158937@google.com>
 <20200424013958.GC158937@google.com>
 <f20ab834-cddb-eaa7-c03e-18f0c4897a33@linux.ibm.com>
 <20200519131009.GD189720@google.com>
 <7c540ac9-ba44-7187-5dc2-60b4c761e91c@linux.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <7c540ac9-ba44-7187-5dc2-60b4c761e91c@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, May 19, 2020 at 03:20:40PM +0200, Laurent Dufour wrote:
> Le 19/05/2020 ? 15:10, Michel Lespinasse a ?crit?:
> > On Mon, May 18, 2020 at 03:45:22PM +0200, Laurent Dufour wrote:
> > > Le 24/04/2020 ? 03:39, Michel Lespinasse a ?crit?:
> > > > Rename the mmap_sem field to mmap_lock. Any new uses of this lock
> > > > should now go through the new mmap locking api. The mmap_lock is
> > > > still implemented as a rwsem, though this could change in the future.
> > > > 
> > > > diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> > > > index dc9ef302f517..701f3995f621 100644
> > > > --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> > > > +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> > > > @@ -661,7 +661,7 @@ static int etnaviv_gem_userptr_get_pages(struct etnaviv_gem_object *etnaviv_obj)
> > > >    	struct etnaviv_gem_userptr *userptr = &etnaviv_obj->userptr;
> > > >    	int ret, pinned = 0, npages = etnaviv_obj->base.size >> PAGE_SHIFT;
> > > > -	might_lock_read(&current->mm->mmap_sem);
> > > > +	might_lock_read(&current->mm->mmap_lock);
> > > 
> > > Why not a mm_might_lock_read() new API to hide the mmap_lock, and add it to
> > > the previous patch?
> > 
> > I'm not sure why this is needed - we may rework the lock to be
> > something else than rwsem, but might_lock_read should still apply to
> > it and make sense ? I'm not sure what the extra API would bring...
> 
> I guess at one time the API would become might_lock_read_a_range(), isn't it?
> Furthermore this would hiding the lock's name which the goal of this series.

I think this assertion should be deleted from this driver.  It's there
in case get_user_pages_fast() takes the mmap sem.  It would make sense to
have this assertion in get_user_pages_fast() in case we take the fast path
which doesn't acquire the mmap_sem.  Something like this:

+++ b/mm/gup.c
@@ -2754,6 +2754,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
                                       FOLL_FORCE | FOLL_PIN | FOLL_GET)))
                return -EINVAL;
 
+       might_lock_read(&current->mm->mmap_lock);
        start = untagged_addr(start) & PAGE_MASK;
        addr = start;
        len = (unsigned long) nr_pages << PAGE_SHIFT;