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Re: [Xen-devel] [PATCH 3 of 9] Enforce ordering constraints for the page alloc lock in the PoD code



Hi, 

The intent of these first three patches looks good to me, but:

- I think it would be better to generate generic spin-lock-with-level
  and unlock-with-level wrapper functions rather than generating the
  various checks and having to assemble them into lock_page_alloc() and
  unlock_page_alloc() by hand.
- p2m->pod.page_alloc_unlock_level is wrong, I think; I can see that you
  need somewhere to store the unlock-level but it shouldn't live in 
  the p2m state - it's at most a per-domain variable, so it should
  live in the struct domain; might as well be beside the lock itself.

Tim.

At 00:33 -0400 on 27 Oct (1319675628), Andres Lagar-Cavilla wrote:
>  xen/arch/x86/mm/mm-locks.h |  11 +++++++++++
>  xen/arch/x86/mm/p2m-pod.c  |  40 +++++++++++++++++++++++++++-------------
>  xen/include/asm-x86/p2m.h  |   5 +++++
>  3 files changed, 43 insertions(+), 13 deletions(-)
> 
> 
> The page alloc lock is sometimes used in the PoD code, with an
> explicit expectation of ordering. Use our ordering constructs in the
> mm layer to enforce this.
> 
> Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
> 
> diff -r c915609e4235 -r 332775f72a30 xen/arch/x86/mm/mm-locks.h
> --- a/xen/arch/x86/mm/mm-locks.h
> +++ b/xen/arch/x86/mm/mm-locks.h
> @@ -155,6 +155,17 @@ declare_mm_lock(p2m)
>  #define p2m_unlock(p)         mm_unlock(&(p)->lock)
>  #define p2m_locked_by_me(p)   mm_locked_by_me(&(p)->lock)
>  
> +/* Page alloc lock (per-domain)
> + *
> + * This is an external lock, not represented by an mm_lock_t. However, 
> + * pod code uses it in conjunction with the p2m lock, and expecting
> + * the ordering which we enforce here */
> +
> +declare_mm_order_constraint(page_alloc)
> +#define page_alloc_mm_pre_lock()   mm_enforce_order_lock_pre_page_alloc()
> +#define page_alloc_mm_post_lock(l) 
> mm_enforce_order_lock_post_page_alloc(&(l))
> +#define page_alloc_mm_unlock(l)    mm_enforce_order_unlock((l))
> +
>  /* Paging lock (per-domain)
>   *
>   * For shadow pagetables, this lock protects
> diff -r c915609e4235 -r 332775f72a30 xen/arch/x86/mm/p2m-pod.c
> --- a/xen/arch/x86/mm/p2m-pod.c
> +++ b/xen/arch/x86/mm/p2m-pod.c
> @@ -45,6 +45,20 @@
>  
>  #define superpage_aligned(_x)  (((_x)&(SUPERPAGE_PAGES-1))==0)
>  
> +/* Enforce lock ordering when grabbing the "external" page_alloc lock */
> +static inline void lock_page_alloc(struct p2m_domain *p2m)
> +{
> +    page_alloc_mm_pre_lock();
> +    spin_lock(&(p2m->domain->page_alloc_lock));
> +    page_alloc_mm_post_lock(p2m->pod.page_alloc_unlock_level);
> +}
> +
> +static inline void unlock_page_alloc(struct p2m_domain *p2m)
> +{
> +    page_alloc_mm_unlock(p2m->pod.page_alloc_unlock_level);
> +    spin_unlock(&(p2m->domain->page_alloc_lock));
> +}
> +
>  /*
>   * Populate-on-demand functionality
>   */
> @@ -100,7 +114,7 @@ p2m_pod_cache_add(struct p2m_domain *p2m
>          unmap_domain_page(b);
>      }
>  
> -    spin_lock(&d->page_alloc_lock);
> +    lock_page_alloc(p2m);
>  
>      /* First, take all pages off the domain list */
>      for(i=0; i < 1 << order ; i++)
> @@ -128,7 +142,7 @@ p2m_pod_cache_add(struct p2m_domain *p2m
>       * This may cause "zombie domains" since the page will never be freed. */
>      BUG_ON( d->arch.relmem != RELMEM_not_started );
>  
> -    spin_unlock(&d->page_alloc_lock);
> +    unlock_page_alloc(p2m);
>  
>      return 0;
>  }
> @@ -245,7 +259,7 @@ p2m_pod_set_cache_target(struct p2m_doma
>  
>          /* Grab the lock before checking that pod.super is empty, or the last
>           * entries may disappear before we grab the lock. */
> -        spin_lock(&d->page_alloc_lock);
> +        lock_page_alloc(p2m);
>  
>          if ( (p2m->pod.count - pod_target) > SUPERPAGE_PAGES
>               && !page_list_empty(&p2m->pod.super) )
> @@ -257,7 +271,7 @@ p2m_pod_set_cache_target(struct p2m_doma
>  
>          ASSERT(page != NULL);
>  
> -        spin_unlock(&d->page_alloc_lock);
> +        unlock_page_alloc(p2m);
>  
>          /* Then free them */
>          for ( i = 0 ; i < (1 << order) ; i++ )
> @@ -378,7 +392,7 @@ p2m_pod_empty_cache(struct domain *d)
>      BUG_ON(!d->is_dying);
>      spin_barrier(&p2m->lock.lock);
>  
> -    spin_lock(&d->page_alloc_lock);
> +    lock_page_alloc(p2m);
>  
>      while ( (page = page_list_remove_head(&p2m->pod.super)) )
>      {
> @@ -403,7 +417,7 @@ p2m_pod_empty_cache(struct domain *d)
>  
>      BUG_ON(p2m->pod.count != 0);
>  
> -    spin_unlock(&d->page_alloc_lock);
> +    unlock_page_alloc(p2m);
>  }
>  
>  int
> @@ -417,7 +431,7 @@ p2m_pod_offline_or_broken_hit(struct pag
>      if ( !(d = page_get_owner(p)) || !(p2m = p2m_get_hostp2m(d)) )
>          return 0;
>  
> -    spin_lock(&d->page_alloc_lock);
> +    lock_page_alloc(p2m);
>      bmfn = mfn_x(page_to_mfn(p));
>      page_list_for_each_safe(q, tmp, &p2m->pod.super)
>      {
> @@ -448,12 +462,12 @@ p2m_pod_offline_or_broken_hit(struct pag
>          }
>      }
>  
> -    spin_unlock(&d->page_alloc_lock);
> +    unlock_page_alloc(p2m);
>      return 0;
>  
>  pod_hit:
>      page_list_add_tail(p, &d->arch.relmem_list);
> -    spin_unlock(&d->page_alloc_lock);
> +    unlock_page_alloc(p2m);
>      return 1;
>  }
>  
> @@ -994,7 +1008,7 @@ p2m_pod_demand_populate(struct p2m_domai
>      if ( q == p2m_guest && gfn > p2m->pod.max_guest )
>          p2m->pod.max_guest = gfn;
>  
> -    spin_lock(&d->page_alloc_lock);
> +    lock_page_alloc(p2m);
>  
>      if ( p2m->pod.count == 0 )
>          goto out_of_memory;
> @@ -1008,7 +1022,7 @@ p2m_pod_demand_populate(struct p2m_domai
>  
>      BUG_ON((mfn_x(mfn) & ((1 << order)-1)) != 0);
>  
> -    spin_unlock(&d->page_alloc_lock);
> +    unlock_page_alloc(p2m);
>  
>      gfn_aligned = (gfn >> order) << order;
>  
> @@ -1040,7 +1054,7 @@ p2m_pod_demand_populate(struct p2m_domai
>  
>      return 0;
>  out_of_memory:
> -    spin_unlock(&d->page_alloc_lock);
> +    unlock_page_alloc(p2m);
>  
>      printk("%s: Out of populate-on-demand memory! tot_pages %" PRIu32 " 
> pod_entries %" PRIi32 "\n",
>             __func__, d->tot_pages, p2m->pod.entry_count);
> @@ -1049,7 +1063,7 @@ out_fail:
>      return -1;
>  remap_and_retry:
>      BUG_ON(order != PAGE_ORDER_2M);
> -    spin_unlock(&d->page_alloc_lock);
> +    unlock_page_alloc(p2m);
>  
>      /* Remap this 2-meg region in singleton chunks */
>      gfn_aligned = (gfn>>order)<<order;
> diff -r c915609e4235 -r 332775f72a30 xen/include/asm-x86/p2m.h
> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -270,6 +270,10 @@ struct p2m_domain {
>       * + p2m_pod_demand_populate() grabs both; the p2m lock to avoid
>       *   double-demand-populating of pages, the page_alloc lock to
>       *   protect moving stuff from the PoD cache to the domain page list.
> +     *
> +     * We enforce this lock ordering through a construct in mm-locks.h.
> +     * This demands, however, that we store the previous lock-ordering
> +     * level in effect before grabbing the page_alloc lock.
>       */
>      struct {
>          struct page_list_head super,   /* List of superpages                
> */
> @@ -279,6 +283,7 @@ struct p2m_domain {
>          unsigned         reclaim_super; /* Last gpfn of a scan */
>          unsigned         reclaim_single; /* Last gpfn of a scan */
>          unsigned         max_guest;    /* gpfn of max guest demand-populate 
> */
> +        int              page_alloc_unlock_level; /* To enforce lock 
> ordering */
>      } pod;
>  };
>  
> 
> _______________________________________________
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> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel

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