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 15:38:24 +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()

> > 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.

> 
> > 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.

Mike
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