linux-kernel - Re: [PATCH 23/26] ib: Convert qib_get_user_pages() to get_user_pages

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Date:	Tue, 8 Oct 2013 21:06:04 +0200
From:	Jan Kara <jack@...e.cz>
To:	"Marciniszyn, Mike" <mike.marciniszyn@...el.com>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	infinipath <infinipath@...el.com>,
	Roland Dreier <roland@...nel.org>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH 23/26] ib: Convert qib_get_user_pages() to
 get_user_pages_unlocked()

On Mon 07-10-13 19:26:04, Jan Kara wrote:
> On Mon 07-10-13 15:38:24, Marciniszyn, Mike wrote:
> > > > This patch and the sibling ipath patch will nominally take the mmap_sem
> > > > twice where the old routine only took it once.   This is a performance
> > > > issue.
> > >   It will take mmap_sem only once during normal operation. Only if
> > > get_user_pages_unlocked() fail, we have to take mmap_sem again to undo
> > > the change of mm->pinned_vm.
> > > 
> > > > Is the intent here to deprecate get_user_pages()?
> > 
> > The old code looked like:
> > __qib_get_user_pages()
> > 	(broken) ulimit test
> >              for (...)
> > 		get_user_pages()
> > 
> > qib_get_user_pages()
> > 	mmap_sem lock
> > 	__qib_get_user_pages()
> >              mmap_sem() unlock
> > 
> > The new code is:
> > 
> > get_user_pages_unlocked()
> > 	mmap_sem  lock
> > 	get_user_pages()
> > 	mmap_sem unlock
> > 
> > qib_get_user_pages()
> > 	mmap_sem lock
> >              ulimit test and locked pages maintenance
> >              mmap_sem unlock
> > 	for (...)
> > 		get_user_pages_unlocked()
> > 
> > I count an additional pair of mmap_sem transactions in the normal case.
>   Ah, sorry, you are right.
> 
> > > > Could the lock limit test be pushed into another version of the
> > > > wrapper so that there is only one set of mmap_sem transactions?
> > >   I'm sorry, I don't understand what you mean here...
> > > 
> > 
> > This is what I had in mind:
> > 
> > get_user_pages_ulimit_unlocked()
> > 	mmap_sem  lock
> > 	ulimit test and locked pages maintenance (from qib/ipath)
> >              for (...)
> > 	       get_user_pages_unlocked()	
> > 	mmap_sem unlock
> > 	
> > qib_get_user_pages()
> > 	get_user_pages_ulimit_unlocked()
> > 
> > This really pushes the code into a new wrapper common to ipath/qib and
> > any others that might want to combine locking with ulimit enforcement.
>   We could do that but frankly, I'd rather change ulimit enforcement to not
> require mmap_sem and use atomic counter instead. I'll see what I can do.
  OK, so something like the attached patch (compile tested only). What do
you think? I'm just not 100% sure removing mmap_sem surrounding stuff like
__ipath_release_user_pages() is safe. I don't see a reason why it shouldn't
be - we have references to the pages and we only mark them dirty and put the
reference - but maybe I miss something subtle...

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

View attachment "0001-mm-Switch-mm-pinned_vm-to-atomic_long_t.patch" of type "text/x-patch" (14826 bytes)