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

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Message-ID: <20131004183315.GA19557@quack.suse.cz>
Date:	Fri, 4 Oct 2013 20:33:15 +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 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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