Re: [Xen-devel] [PATCH 1/2] xen/p2m: Fix for 32-bit builds the "Reserve 8MB of _brk space for P2M"

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Aug 17, 2012 at 12:14:12PM +0100, David Vrabel wrote:
> On 16/08/12 22:02, Konrad Rzeszutek Wilk wrote:
> > 
> > So I thought about this some more and came up with this patch. Its
> > RFC and going to run it through some overnight tests to see how they fare.
> > 
> > 
> > commit da858a92dbeb52fb3246e3d0f1dd57989b5b1734
> > Author: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > Date:   Fri Jul 27 16:05:47 2012 -0400
> > 
> >     xen/p2m: Reuse existing P2M leafs if they are filled with 1:1 PFNs or INVALID.
> >     
> >     If P2M leaf is completly packed with INVALID_P2M_ENTRY or with
> >     1:1 PFNs (so IDENTITY_FRAME type PFNs), we can swap the P2M leaf
> >     with either a p2m_missing or p2m_identity respectively. The old
> >     page (which was created via extend_brk or was grafted on from the
> >     mfn_list) can be re-used for setting new PFNs.
> 
> Does this actually find any p2m pages to reclaim?

Very much so. When I run the kernel without dom0_mem, and end up returning
around 372300 pages back, and then populating them back - they (mostly)
all get to re-use the transplanted mfn_list.

The ones in the 9a-100 obviously don't.
> 
> xen_set_identity_and_release() is careful to set the largest possible
> range as 1:1 and the comments at the top of p2m.c suggest the mid
> entries will be made to point to p2m_identity already.

Right, and that is still true - for cases where the are no mid entries
(so P2M[3][400] for example can point in the middle of the MMIO region).

But if you boot without dom0_mem=max, that region (P2M[3][400]) would at
the start be backed by the &mfn_list, so when we call 1-1 on that region
it ends up sticking in the &mfn_list a whole bunch of IDENTITY_FRAME(pfn).

This patch harvests those chunks of &mfn_list that have that and re-uses them.

And without any dom0_mem= I seem to at most call extend_bkr twice (to
allocate the top leafs P2M[4] and P2M[5]). Hm, to be on a safe side I should
probably do 'reserve_brk(p2m_popualated, 3 * PAGE_SIZE)' in case we
end up transplanting 3GB of PFNs in in the P2M[4], P2M[5] and P2M[6] nodes.

> 
> David
> 
> >     This also means we can remove git commit:
> >     5bc6f9888db5739abfa0cae279b4b442e4db8049
> >     xen/p2m: Reserve 8MB of _brk space for P2M leafs when populating back
> >     which tried to fix this.
> >     
> >     Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > 
> > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> > index 29244d0..b6b7c10 100644
> > --- a/arch/x86/xen/p2m.c
> > +++ b/arch/x86/xen/p2m.c
> > @@ -194,11 +194,6 @@ RESERVE_BRK(p2m_mid_mfn, PAGE_SIZE * (MAX_DOMAIN_PAGES / (P2M_PER_PAGE * P2M_MID
> >   * boundary violation will require three middle nodes. */
> >  RESERVE_BRK(p2m_mid_identity, PAGE_SIZE * 2 * 3);
> >  
> > -/* When we populate back during bootup, the amount of pages can vary. The
> > - * max we have is seen is 395979, but that does not mean it can't be more.
> > - * Some machines can have 3GB I/O holes so lets reserve for that. */
> > -RESERVE_BRK(p2m_populated, 786432 * sizeof(unsigned long));
> > -
> >  static inline unsigned p2m_top_index(unsigned long pfn)
> >  {
> >  	BUG_ON(pfn >= MAX_P2M_PFN);
> > @@ -575,12 +570,99 @@ static bool __init early_alloc_p2m(unsigned long pfn)
> >  	}
> >  	return true;
> >  }
> > +
> > +/*
> > + * Skim over the P2M tree looking at pages that are either filled with
> > + * INVALID_P2M_ENTRY or with 1:1 PFNs. If found, re-use that page and
> > + * replace the P2M leaf with a p2m_missing or p2m_identity.
> > + * Stick the old page in the new P2M tree location.
> > + */
> > +bool __init early_can_reuse_p2m_middle(unsigned long set_pfn, unsigned long set_mfn)
> > +{
> > +	unsigned topidx;
> > +	unsigned mididx;
> > +	unsigned ident_pfns;
> > +	unsigned inv_pfns;
> > +	unsigned long *p2m;
> > +	unsigned long *mid_mfn_p;
> > +	unsigned idx;
> > +	unsigned long pfn;
> > +
> > +	/* We only look when this entails a P2M middle layer */
> > +	if (p2m_index(set_pfn))
> > +		return false;
> > +
> > +	for (pfn = 0; pfn <= MAX_DOMAIN_PAGES; pfn += P2M_PER_PAGE) {
> > +		topidx = p2m_top_index(pfn);
> > +
> > +		if (!p2m_top[topidx])
> > +			continue;
> > +
> > +		if (p2m_top[topidx] == p2m_mid_missing)
> > +			continue;
> > +
> > +		mididx = p2m_mid_index(pfn);
> > +		p2m = p2m_top[topidx][mididx];
> > +		if (!p2m)
> > +			continue;
> > +
> > +		if ((p2m == p2m_missing) || (p2m == p2m_identity))
> > +			continue;
> > +
> > +		if ((unsigned long)p2m == INVALID_P2M_ENTRY)
> > +			continue;
> > +
> > +		ident_pfns = 0;
> > +		inv_pfns = 0;
> > +		for (idx = 0; idx < P2M_PER_PAGE; idx++) {
> > +			/* IDENTITY_PFNs are 1:1 */
> > +			if (p2m[idx] == IDENTITY_FRAME(pfn + idx))
> > +				ident_pfns++;
> > +			else if (p2m[idx] == INVALID_P2M_ENTRY)
> > +				inv_pfns++;
> > +			else
> > +				break;
> > +		}
> > +		if ((ident_pfns == P2M_PER_PAGE) || (inv_pfns == P2M_PER_PAGE))
> > +			goto found;
> > +	}
> > +	return false;
> > +found:
> > +	/* Found one, replace old with p2m_identity or p2m_missing */
> > +	p2m_top[topidx][mididx] = (ident_pfns ? p2m_identity : p2m_missing);
> > +	/* And the other for save/restore.. */
> > +	mid_mfn_p = p2m_top_mfn_p[topidx];
> > +	/* NOTE: Even if it is a p2m_identity it should still be point to
> > +	 * a page filled with INVALID_P2M_ENTRY entries. */
> > +	mid_mfn_p[mididx] = virt_to_mfn(p2m_missing);
> > +
> > +	/* Reset where we want to stick the old page in. */
> > +	topidx = p2m_top_index(set_pfn);
> > +	mididx = p2m_mid_index(set_pfn);
> > +
> > +	/* This shouldn't happen */
> > +	if (WARN_ON(p2m_top[topidx] == p2m_mid_missing))
> > +		early_alloc_p2m(set_pfn);
> > +
> > +	if (WARN_ON(p2m_top[topidx][mididx] != p2m_missing))
> > +		return false;
> > +
> > +	p2m_init(p2m);
> > +	p2m_top[topidx][mididx] = p2m;
> > +	mid_mfn_p = p2m_top_mfn_p[topidx];
> > +	mid_mfn_p[mididx] = virt_to_mfn(p2m);
> > +
> > +	return true;
> > +}
> >  bool __init early_set_phys_to_machine(unsigned long pfn, unsigned long mfn)
> >  {
> >  	if (unlikely(!__set_phys_to_machine(pfn, mfn)))  {
> >  		if (!early_alloc_p2m(pfn))
> >  			return false;
> >  
> > +		if (early_can_reuse_p2m_middle(pfn, mfn))
> > +			return __set_phys_to_machine(pfn, mfn);
> > +
> >  		if (!early_alloc_p2m_middle(pfn, false /* boundary crossover OK!*/))
> >  			return false;
> >  
> > 
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