Re: [PATCH v5.5 10/10] mmap locking API: rename mmap_sem to mmap_lock

From: John Hubbard <jhubbard@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>,
	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>
Subject: Re: [PATCH v5.5 10/10] mmap locking API: rename mmap_sem to mmap_lock
Date: Tue, 19 May 2020 11:14:57 -0700	[thread overview]
Message-ID: <10d48b77-5c6e-2e10-84e6-16cdd76a45f1@nvidia.com> (raw)
In-Reply-To: <20200519153251.GY16070@bombadil.infradead.org>

On 2020-05-19 08:32, Matthew Wilcox wrote:
> On Tue, May 19, 2020 at 03:20:40PM +0200, Laurent Dufour wrote:
>> Le 19/05/2020 à 15:10, Michel Lespinasse a écrit :
>>> On Mon, May 18, 2020 at 03:45:22PM +0200, Laurent Dufour wrote:
>>>> Le 24/04/2020 à 03:39, Michel Lespinasse a écrit :
>>>>> 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;
> 
> 

Hi Michel and Matthew and all,

There are a couple of recent developments in this code to keep in mind. I don't
*think* either one is a problem here, but just in case:

a) The latest version of the above routine [1] is on its way to mmotm as of
yesterday, and that version more firmly divides the fast and slow parts,
via a new FOLL_FAST_ONLY flag. The fall-back to slow/regular gup only occurs
if the caller does not set FOLL_FAST_ONLY. (Note that it's a gup.c internal
flag, btw.)

That gives you additional options inside internal_get_user_pages_fast(), such
as, approximately:

if (!(gup_flags & FOLL_FAST_ONLY))
	might_lock_read(&current->mm->mmap_lock);

...not that that is necessarily a great idea, seeing as how it merely changes
"might lock" into "maybe might lock".  :)

b) I've posted a small patch to that same etnaviv_gem.c file [2], over the
weekend, to convert from get_user_pages()/put_page(), to 
pin_user_pages()/unpin_user_pages(). It hasn't been merged yet, and it
shouldn't conflict either, but just one more reason to hope that the
etnaviv list can do some run time testing on the whole lot.

[1] https://lore.kernel.org/r/20200519002124.2025955-3-jhubbard@nvidia.com

[2] https://lore.kernel.org/r/20200518054315.2407093-1-jhubbard@nvidia.com

thanks,
-- 
John Hubbard
NVIDIA