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Date: Fri, 17 Aug 2012 09:06:21 -0400
From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To: David Vrabel <david.vrabel@...rix.com>
Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com
Subject: Re: [Xen-devel] [PATCH 1/2] xen/p2m: Fix for 32-bit builds the
"Reserve 8MB of _brk space for P2M"
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@...cle.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@...cle.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;
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@...ts.xen.org
> > http://lists.xen.org/xen-devel
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