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

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Message-ID: <20131007172604.GD30441@quack.suse.cz>
Date:	Mon, 7 Oct 2013 19:26:04 +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 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.

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