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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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@xxxxxxxxxx>
> 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?

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.

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@xxxxxxxxxx>
> 
> 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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