Re: [RFC][PATCH 4/8] mm: RCU free vmas

[Paul E. McKenney]

On Mon, Jan 04, 2010 at 07:24:33PM +0100, Peter Zijlstra wrote:
> TODO:
>  - should be SRCU, lack of call_srcu()
> 
> In order to allow speculative vma lookups, RCU free the struct
> vm_area_struct.
> 
> We use two means of detecting a vma is still valid:
>  - firstly, we set RB_CLEAR_NODE once we remove a vma from the tree.
>  - secondly, we check the vma sequence number.
> 
> These two things combined will guarantee that 1) the vma is still
> present and two, it still covers the same range from when we looked it
> up.

OK, I think I see what you are up to here.  I could get you a very crude
throw-away call_srcu() fairly quickly.  I don't yet have a good estimate
of how long it will take me to merge SRCU into the treercu infrastructure,
but am getting there.

So, which release are you thinking in terms of?

							Thanx, Paul

> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
>  include/linux/mm.h       |   12 ++++++++++++
>  include/linux/mm_types.h |    2 ++
>  init/Kconfig             |   34 +++++++++++++++++-----------------
>  kernel/sched.c           |    9 ++++++++-
>  mm/mmap.c                |   33 +++++++++++++++++++++++++++++++--
>  5 files changed, 70 insertions(+), 20 deletions(-)
> 
> Index: linux-2.6/include/linux/mm.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mm.h
> +++ linux-2.6/include/linux/mm.h
> @@ -765,6 +765,18 @@ unsigned long unmap_vmas(struct mmu_gath
>  		unsigned long end_addr, unsigned long *nr_accounted,
>  		struct zap_details *);
> 
> +static inline int vma_is_dead(struct vm_area_struct *vma, unsigned int sequence)
> +{
> +	int ret = RB_EMPTY_NODE(&vma->vm_rb);
> +	unsigned seq = vma->vm_sequence.sequence;
> +	/*
> +	 * Matches both the wmb in write_seqlock_begin/end() and
> +	 * the wmb in detach_vmas_to_be_unmapped()/__unlink_vma().
> +	 */
> +	smp_rmb();
> +	return ret || seq != sequence;
> +}
> +
>  /**
>   * mm_walk - callbacks for walk_page_range
>   * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
> Index: linux-2.6/include/linux/mm_types.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mm_types.h
> +++ linux-2.6/include/linux/mm_types.h
> @@ -13,6 +13,7 @@
>  #include <linux/cpumask.h>
>  #include <linux/page-debug-flags.h>
>  #include <linux/seqlock.h>
> +#include <linux/rcupdate.h>
>  #include <asm/page.h>
>  #include <asm/mmu.h>
> 
> @@ -188,6 +189,7 @@ struct vm_area_struct {
>  	struct mempolicy *vm_policy;	/* NUMA policy for the VMA */
>  #endif
>  	seqcount_t vm_sequence;
> +	struct rcu_head vm_rcu_head;
>  };
> 
>  struct core_thread {
> Index: linux-2.6/mm/mmap.c
> ===================================================================
> --- linux-2.6.orig/mm/mmap.c
> +++ linux-2.6/mm/mmap.c
> @@ -222,6 +222,19 @@ void unlink_file_vma(struct vm_area_stru
>  	}
>  }
> 
> +static void free_vma_rcu(struct rcu_head *head)
> +{
> +	struct vm_area_struct *vma =
> +		container_of(head, struct vm_area_struct, vm_rcu_head);
> +
> +	kmem_cache_free(vm_area_cachep, vma);
> +}
> +
> +static void free_vma(struct vm_area_struct *vma)
> +{
> +	call_rcu(&vma->vm_rcu_head, free_vma_rcu);
> +}
> +
>  /*
>   * Close a vm structure and free it, returning the next.
>   */
> @@ -238,7 +251,7 @@ static struct vm_area_struct *remove_vma
>  			removed_exe_file_vma(vma->vm_mm);
>  	}
>  	mpol_put(vma_policy(vma));
> -	kmem_cache_free(vm_area_cachep, vma);
> +	free_vma(vma);
>  	return next;
>  }
> 
> @@ -488,6 +501,14 @@ __vma_unlink(struct mm_struct *mm, struc
>  {
>  	prev->vm_next = vma->vm_next;
>  	rb_erase(&vma->vm_rb, &mm->mm_rb);
> +	/*
> +	 * Ensure the removal is completely comitted to memory
> +	 * before clearing the node.
> +	 *
> +	 * Matched by vma_is_dead()/handle_speculative_fault().
> +	 */
> +	smp_wmb();
> +	RB_CLEAR_NODE(&vma->vm_rb);
>  	if (mm->mmap_cache == vma)
>  		mm->mmap_cache = prev;
>  }
> @@ -644,7 +665,7 @@ again:			remove_next = 1 + (end > next->
>  		}
>  		mm->map_count--;
>  		mpol_put(vma_policy(next));
> -		kmem_cache_free(vm_area_cachep, next);
> +		free_vma(next);
>  		/*
>  		 * In mprotect's case 6 (see comments on vma_merge),
>  		 * we must remove another next too. It would clutter
> @@ -1858,6 +1879,14 @@ detach_vmas_to_be_unmapped(struct mm_str
>  	insertion_point = (prev ? &prev->vm_next : &mm->mmap);
>  	do {
>  		rb_erase(&vma->vm_rb, &mm->mm_rb);
> +		/*
> +		 * Ensure the removal is completely comitted to memory
> +		 * before clearing the node.
> +		 *
> +		 * Matched by vma_is_dead()/handle_speculative_fault().
> +		 */
> +		smp_wmb();
> +		RB_CLEAR_NODE(&vma->vm_rb);
>  		mm->map_count--;
>  		tail_vma = vma;
>  		vma = vma->vm_next;
> Index: linux-2.6/init/Kconfig
> ===================================================================
> --- linux-2.6.orig/init/Kconfig
> +++ linux-2.6/init/Kconfig
> @@ -314,19 +314,19 @@ menu "RCU Subsystem"
> 
>  choice
>  	prompt "RCU Implementation"
> -	default TREE_RCU
> +	default TREE_PREEMPT_RCU
> 
> -config TREE_RCU
> -	bool "Tree-based hierarchical RCU"
> -	help
> -	  This option selects the RCU implementation that is
> -	  designed for very large SMP system with hundreds or
> -	  thousands of CPUs.  It also scales down nicely to
> -	  smaller systems.
> +#config TREE_RCU
> +#	bool "Tree-based hierarchical RCU"
> +#	help
> +#	  This option selects the RCU implementation that is
> +#	  designed for very large SMP system with hundreds or
> +#	  thousands of CPUs.  It also scales down nicely to
> +#	  smaller systems.
> 
>  config TREE_PREEMPT_RCU
>  	bool "Preemptable tree-based hierarchical RCU"
> -	depends on PREEMPT
> +#	depends on PREEMPT
>  	help
>  	  This option selects the RCU implementation that is
>  	  designed for very large SMP systems with hundreds or
> @@ -334,14 +334,14 @@ config TREE_PREEMPT_RCU
>  	  is also required.  It also scales down nicely to
>  	  smaller systems.
> 
> -config TINY_RCU
> -	bool "UP-only small-memory-footprint RCU"
> -	depends on !SMP
> -	help
> -	  This option selects the RCU implementation that is
> -	  designed for UP systems from which real-time response
> -	  is not required.  This option greatly reduces the
> -	  memory footprint of RCU.
> +#config TINY_RCU
> +#	bool "UP-only small-memory-footprint RCU"
> +#	depends on !SMP
> +#	help
> +#	  This option selects the RCU implementation that is
> +#	  designed for UP systems from which real-time response
> +#	  is not required.  This option greatly reduces the
> +#	  memory footprint of RCU.
> 
>  endchoice
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -9689,7 +9689,14 @@ void __init sched_init(void)
>  #ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
>  static inline int preempt_count_equals(int preempt_offset)
>  {
> -	int nested = (preempt_count() & ~PREEMPT_ACTIVE) + rcu_preempt_depth();
> +	int nested = (preempt_count() & ~PREEMPT_ACTIVE)
> +		/*
> +		 * remove this for we need preemptible RCU
> +		 * exactly because it needs to sleep..
> +		 *
> +		 + rcu_preempt_depth()
> +		 */
> +		;
> 
>  	return (nested == PREEMPT_INATOMIC_BASE + preempt_offset);
>  }
> 
> -- 
> 
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