linux-kernel - Re: [PATCH 3/8] mm, hugetlb: fix race in region tracking

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Message-ID: <1390958378.11839.37.camel@buesod1.americas.hpqcorp.net>
Date:	Tue, 28 Jan 2014 17:19:38 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	akpm@...ux-foundation.org, iamjoonsoo.kim@....com, riel@...hat.com,
	mgorman@...e.de, mhocko@...e.cz, aneesh.kumar@...ux.vnet.ibm.com,
	kamezawa.hiroyu@...fujitsu.com, hughd@...gle.com,
	david@...son.dropbear.id.au, js1304@...il.com,
	liwanp@...ux.vnet.ibm.com, dhillf@...il.com, rientjes@...gle.com,
	aswin@...com, scott.norton@...com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] mm, hugetlb: fix race in region tracking

On Tue, 2014-01-28 at 19:36 -0500, Naoya Horiguchi wrote:
> On Mon, Jan 27, 2014 at 06:34:17PM -0800, Davidlohr Bueso wrote:
[...]
> > > If this retry is really essential for the fix, please comment the reason
> > > both in patch description and inline comment. It's very important for
> > > future code maintenance.
> > 
> > So we locate the corresponding region in the reserve map, and if we are
> > below the current region, then we allocate a new one. Since we dropped
> > the lock to allocate memory, we have to make sure that we still need the
> > new region and that we don't race with the new status of the reservation
> > map. This is the whole point of the retry, and I don't see it being
> > suboptimal.
> 
> I'm afraid that you don't explain why you need drop the lock for memory
> allocation. Are you saying that this unlocking comes from the difference
> between rwsem and spin lock?

Because you cannot go to sleep while holding a spinlock, which is
exactly what kmalloc(GFP_KERNEL) can do. We *might* get a way with it
with GFP_ATOMIC, I dunno, but I certainly prefer this approach better.

Thanks,
Davidlohr

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