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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 28 Jan 2014 17:19:38 -0800 Davidlohr Bueso <davidlohr@hp.com> wrote:

> 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.

yup.  You could do

	foo = kmalloc(size, GFP_NOWAIT);
	if (!foo) {
		spin_unlock(...);
		foo = kmalloc(size, GFP_KERNEL);
		if (!foo)
			...
		spin_lock(...);
	}

that avoids the lock/unlock once per allocation.  But it also increases
the lock's average hold times....

		

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