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

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Date:	Mon, 7 Oct 2013 15:20:28 +0000
From:	"Marciniszyn, Mike" <mike.marciniszyn@...el.com>
To:	Jan Kara <jack@...e.cz>
CC:	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()



> -----Original Message-----
> From: Jan Kara [mailto:jack@...e.cz]
> Sent: Friday, October 04, 2013 2:33 PM
> To: Marciniszyn, Mike
> Cc: Jan Kara; LKML; linux-mm@...ck.org; infinipath; Roland Dreier; linux-
> rdma@...r.kernel.org
> Subject: Re: [PATCH 23/26] ib: Convert qib_get_user_pages() to
> get_user_pages_unlocked()
> 
> On Fri 04-10-13 13:52:49, Marciniszyn, Mike wrote:
> > > Convert qib_get_user_pages() to use get_user_pages_unlocked().  This
> > > shortens the section where we hold mmap_sem for writing and also
> > > removes the knowledge about get_user_pages() locking from ipath
> > > driver. We also fix a bug in testing pinned number of pages when
> changing the code.
> > >
> >
> > 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()?
>   Well, as much as I'd like to, there are really places in mm code which need
> to call get_user_pages() while holding mmap_sem to be able to inspect
> corresponding vmas etc. So I want to reduce get_user_pages() use as much
> as possible but I'm not really hoping in completely removing it.
> 
> > I agree, the old code's lock limit test is broke and needs to be fixed.
> > I like the elimination of the silly wrapper routine!
> >
> > 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...
> 
> 								Honza
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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