linux-kernel - Re: [PATCH 2/2] hugetlb reservations: fix hugetlb MAP

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Date:	Mon, 23 Jun 2008 10:53:10 +0100
From:	Andy Whitcroft <apw@...dowen.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Jon Tollefson <kniht@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Nishanth Aravamudan <nacc@...ibm.com>,
	Adam Litke <agl@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/2] hugetlb reservations: fix hugetlb MAP_PRIVATE reservations across vma splits

On Mon, Jun 23, 2008 at 09:00:48AM +0100, Mel Gorman wrote:
> Typical. I spotted this after I pushed send.....
> 
> > <SNIP>
> 
> > @@ -266,14 +326,19 @@ static void decrement_hugepage_resv_vma(struct hstate *h,
> >  		 * private mappings.
> >  		 */
> >  		if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
> > -			unsigned long flags, reserve;
> > +			unsigned long idx = vma_pagecache_offset(h,
> > +							vma, address);
> > +			struct resv_map *reservations = vma_resv_map(vma);
> > +
> >  			h->resv_huge_pages--;
> > -			flags = (unsigned long)vma->vm_private_data &
> > -							HPAGE_RESV_MASK;
> > -			reserve = (unsigned long)vma->vm_private_data - 1;
> > -			vma->vm_private_data = (void *)(reserve | flags);
> > +
> > +			/* Mark this page used in the map. */
> > +			if (region_chg(&reservations->regions, idx, idx + 1) < 0)
> > +				return -1;
> > +			region_add(&reservations->regions, idx, idx + 1);
> >  		}
> 
> decrement_hugepage_resv_vma() is called with hugetlb_lock held and region_chg
> calls kmalloc(GFP_KERNEL).  Hence it's possible we would sleep with that
> spinlock held which is a bit uncool. The allocation needs to happen outside
> the lock. Right?

Yes, good spot.  Luckily this pair of calls can be separated, as the
first is a prepare and the second a commit.  So I can trivially pull
the allocation outside the lock.

Had a quick go at this and it looks like I can move both out of the lock
to a much more logical spot and clean the patch up significantly.  Will
fold in your other comments and post up a V2 once it has been tested.

Thanks.

-apw
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