Re: [PATCH v2 4/6] mm, mempolicy: simplify rebinding mempolicies when updating cpusets

[Michal Hocko <mhocko@kernel.org>]

On Wed 17-05-17 10:11:38, Vlastimil Babka wrote:
> Commit c0ff7453bb5c ("cpuset,mm: fix no node to alloc memory when changing
> cpuset's mems") has introduced a two-step protocol when rebinding task's
> mempolicy due to cpuset update, in order to avoid a parallel allocation seeing
> an empty effective nodemask and failing. Later, commit cc9a6c877661 ("cpuset:
> mm: reduce large amounts of memory barrier related damage v3") introduced
> a seqlock protection and removed the synchronization point between the two
> update steps. At that point (or perhaps later), the two-step rebinding became
> unnecessary. Currently it only makes sure that the update first adds new nodes
> in step 1 and then removes nodes in step 2. Without memory barriers the effects
> are questionable, and even then this cannot prevent a parallel zonelist
> iteration checking the nodemask at each step to observe all nodes as unusable
> for allocation. We now fully rely on the seqlock to prevent premature OOMs and
> allocation failures.
> 
> We can thus remove the two-step update parts and simplify the code.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

\o/ This code has been just piling up more complicated code on top of
the code which shouldn't have existed in the first place so I am very
happy to see it go. I hope we can go without rebinding altogether
longterm. Chaging data under feets just asks for problems and this is a
nice example of where it goes.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  include/linux/mempolicy.h      |   6 +--
>  include/uapi/linux/mempolicy.h |   8 ----
>  kernel/cgroup/cpuset.c         |   4 +-
>  mm/mempolicy.c                 | 102 ++++++++---------------------------------
>  4 files changed, 21 insertions(+), 99 deletions(-)
> 
> diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
> index ecb6cbeede5a..3a58b4be1b0c 100644
> --- a/include/linux/mempolicy.h
> +++ b/include/linux/mempolicy.h
> @@ -142,8 +142,7 @@ bool vma_policy_mof(struct vm_area_struct *vma);
>  
>  extern void numa_default_policy(void);
>  extern void numa_policy_init(void);
> -extern void mpol_rebind_task(struct task_struct *tsk, const nodemask_t *new,
> -				enum mpol_rebind_step step);
> +extern void mpol_rebind_task(struct task_struct *tsk, const nodemask_t *new);
>  extern void mpol_rebind_mm(struct mm_struct *mm, nodemask_t *new);
>  
>  extern int huge_node(struct vm_area_struct *vma,
> @@ -260,8 +259,7 @@ static inline void numa_default_policy(void)
>  }
>  
>  static inline void mpol_rebind_task(struct task_struct *tsk,
> -				const nodemask_t *new,
> -				enum mpol_rebind_step step)
> +				const nodemask_t *new)
>  {
>  }
>  
> diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
> index 9cd8b21dddbe..2a4d89508fec 100644
> --- a/include/uapi/linux/mempolicy.h
> +++ b/include/uapi/linux/mempolicy.h
> @@ -24,13 +24,6 @@ enum {
>  	MPOL_MAX,	/* always last member of enum */
>  };
>  
> -enum mpol_rebind_step {
> -	MPOL_REBIND_ONCE,	/* do rebind work at once(not by two step) */
> -	MPOL_REBIND_STEP1,	/* first step(set all the newly nodes) */
> -	MPOL_REBIND_STEP2,	/* second step(clean all the disallowed nodes)*/
> -	MPOL_REBIND_NSTEP,
> -};
> -
>  /* Flags for set_mempolicy */
>  #define MPOL_F_STATIC_NODES	(1 << 15)
>  #define MPOL_F_RELATIVE_NODES	(1 << 14)
> @@ -65,7 +58,6 @@ enum mpol_rebind_step {
>   */
>  #define MPOL_F_SHARED  (1 << 0)	/* identify shared policies */
>  #define MPOL_F_LOCAL   (1 << 1)	/* preferred local allocation */
> -#define MPOL_F_REBINDING (1 << 2)	/* identify policies in rebinding */
>  #define MPOL_F_MOF	(1 << 3) /* this policy wants migrate on fault */
>  #define MPOL_F_MORON	(1 << 4) /* Migrate On protnone Reference On Node */
>  
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 0f41292be0fb..dfd5b420452d 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1063,9 +1063,7 @@ static void cpuset_change_task_nodemask(struct task_struct *tsk,
>  	}
>  
>  	nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);
> -	mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP1);
> -
> -	mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP2);
> +	mpol_rebind_task(tsk, newmems);
>  	tsk->mems_allowed = *newmems;
>  
>  	if (need_loop) {
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index c60807625fd5..047181452040 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -146,22 +146,7 @@ struct mempolicy *get_task_policy(struct task_struct *p)
>  
>  static const struct mempolicy_operations {
>  	int (*create)(struct mempolicy *pol, const nodemask_t *nodes);
> -	/*
> -	 * If read-side task has no lock to protect task->mempolicy, write-side
> -	 * task will rebind the task->mempolicy by two step. The first step is
> -	 * setting all the newly nodes, and the second step is cleaning all the
> -	 * disallowed nodes. In this way, we can avoid finding no node to alloc
> -	 * page.
> -	 * If we have a lock to protect task->mempolicy in read-side, we do
> -	 * rebind directly.
> -	 *
> -	 * step:
> -	 * 	MPOL_REBIND_ONCE - do rebind work at once
> -	 * 	MPOL_REBIND_STEP1 - set all the newly nodes
> -	 * 	MPOL_REBIND_STEP2 - clean all the disallowed nodes
> -	 */
> -	void (*rebind)(struct mempolicy *pol, const nodemask_t *nodes,
> -			enum mpol_rebind_step step);
> +	void (*rebind)(struct mempolicy *pol, const nodemask_t *nodes);
>  } mpol_ops[MPOL_MAX];
>  
>  static inline int mpol_store_user_nodemask(const struct mempolicy *pol)
> @@ -304,19 +289,11 @@ void __mpol_put(struct mempolicy *p)
>  	kmem_cache_free(policy_cache, p);
>  }
>  
> -static void mpol_rebind_default(struct mempolicy *pol, const nodemask_t *nodes,
> -				enum mpol_rebind_step step)
> +static void mpol_rebind_default(struct mempolicy *pol, const nodemask_t *nodes)
>  {
>  }
>  
> -/*
> - * step:
> - * 	MPOL_REBIND_ONCE  - do rebind work at once
> - * 	MPOL_REBIND_STEP1 - set all the newly nodes
> - * 	MPOL_REBIND_STEP2 - clean all the disallowed nodes
> - */
> -static void mpol_rebind_nodemask(struct mempolicy *pol, const nodemask_t *nodes,
> -				 enum mpol_rebind_step step)
> +static void mpol_rebind_nodemask(struct mempolicy *pol, const nodemask_t *nodes)
>  {
>  	nodemask_t tmp;
>  
> @@ -325,35 +302,19 @@ static void mpol_rebind_nodemask(struct mempolicy *pol, const nodemask_t *nodes,
>  	else if (pol->flags & MPOL_F_RELATIVE_NODES)
>  		mpol_relative_nodemask(&tmp, &pol->w.user_nodemask, nodes);
>  	else {
> -		/*
> -		 * if step == 1, we use ->w.cpuset_mems_allowed to cache the
> -		 * result
> -		 */
> -		if (step == MPOL_REBIND_ONCE || step == MPOL_REBIND_STEP1) {
> -			nodes_remap(tmp, pol->v.nodes,
> -					pol->w.cpuset_mems_allowed, *nodes);
> -			pol->w.cpuset_mems_allowed = step ? tmp : *nodes;
> -		} else if (step == MPOL_REBIND_STEP2) {
> -			tmp = pol->w.cpuset_mems_allowed;
> -			pol->w.cpuset_mems_allowed = *nodes;
> -		} else
> -			BUG();
> +		nodes_remap(tmp, pol->v.nodes,pol->w.cpuset_mems_allowed,
> +								*nodes);
> +		pol->w.cpuset_mems_allowed = tmp;
>  	}
>  
>  	if (nodes_empty(tmp))
>  		tmp = *nodes;
>  
> -	if (step == MPOL_REBIND_STEP1)
> -		nodes_or(pol->v.nodes, pol->v.nodes, tmp);
> -	else if (step == MPOL_REBIND_ONCE || step == MPOL_REBIND_STEP2)
> -		pol->v.nodes = tmp;
> -	else
> -		BUG();
> +	pol->v.nodes = tmp;
>  }
>  
>  static void mpol_rebind_preferred(struct mempolicy *pol,
> -				  const nodemask_t *nodes,
> -				  enum mpol_rebind_step step)
> +						const nodemask_t *nodes)
>  {
>  	nodemask_t tmp;
>  
> @@ -379,42 +340,19 @@ static void mpol_rebind_preferred(struct mempolicy *pol,
>  /*
>   * mpol_rebind_policy - Migrate a policy to a different set of nodes
>   *
> - * If read-side task has no lock to protect task->mempolicy, write-side
> - * task will rebind the task->mempolicy by two step. The first step is
> - * setting all the newly nodes, and the second step is cleaning all the
> - * disallowed nodes. In this way, we can avoid finding no node to alloc
> - * page.
> - * If we have a lock to protect task->mempolicy in read-side, we do
> - * rebind directly.
> - *
> - * step:
> - * 	MPOL_REBIND_ONCE  - do rebind work at once
> - * 	MPOL_REBIND_STEP1 - set all the newly nodes
> - * 	MPOL_REBIND_STEP2 - clean all the disallowed nodes
> + * Per-vma policies are protected by mmap_sem. Allocations using per-task
> + * policies are protected by task->mems_allowed_seq to prevent a premature
> + * OOM/allocation failure due to parallel nodemask modification.
>   */
> -static void mpol_rebind_policy(struct mempolicy *pol, const nodemask_t *newmask,
> -				enum mpol_rebind_step step)
> +static void mpol_rebind_policy(struct mempolicy *pol, const nodemask_t *newmask)
>  {
>  	if (!pol)
>  		return;
> -	if (!mpol_store_user_nodemask(pol) && step == MPOL_REBIND_ONCE &&
> +	if (!mpol_store_user_nodemask(pol) &&
>  	    nodes_equal(pol->w.cpuset_mems_allowed, *newmask))
>  		return;
>  
> -	if (step == MPOL_REBIND_STEP1 && (pol->flags & MPOL_F_REBINDING))
> -		return;
> -
> -	if (step == MPOL_REBIND_STEP2 && !(pol->flags & MPOL_F_REBINDING))
> -		BUG();
> -
> -	if (step == MPOL_REBIND_STEP1)
> -		pol->flags |= MPOL_F_REBINDING;
> -	else if (step == MPOL_REBIND_STEP2)
> -		pol->flags &= ~MPOL_F_REBINDING;
> -	else if (step >= MPOL_REBIND_NSTEP)
> -		BUG();
> -
> -	mpol_ops[pol->mode].rebind(pol, newmask, step);
> +	mpol_ops[pol->mode].rebind(pol, newmask);
>  }
>  
>  /*
> @@ -424,10 +362,9 @@ static void mpol_rebind_policy(struct mempolicy *pol, const nodemask_t *newmask,
>   * Called with task's alloc_lock held.
>   */
>  
> -void mpol_rebind_task(struct task_struct *tsk, const nodemask_t *new,
> -			enum mpol_rebind_step step)
> +void mpol_rebind_task(struct task_struct *tsk, const nodemask_t *new)
>  {
> -	mpol_rebind_policy(tsk->mempolicy, new, step);
> +	mpol_rebind_policy(tsk->mempolicy, new);
>  }
>  
>  /*
> @@ -442,7 +379,7 @@ void mpol_rebind_mm(struct mm_struct *mm, nodemask_t *new)
>  
>  	down_write(&mm->mmap_sem);
>  	for (vma = mm->mmap; vma; vma = vma->vm_next)
> -		mpol_rebind_policy(vma->vm_policy, new, MPOL_REBIND_ONCE);
> +		mpol_rebind_policy(vma->vm_policy, new);
>  	up_write(&mm->mmap_sem);
>  }
>  
> @@ -2101,10 +2038,7 @@ struct mempolicy *__mpol_dup(struct mempolicy *old)
>  
>  	if (current_cpuset_is_being_rebound()) {
>  		nodemask_t mems = cpuset_mems_allowed(current);
> -		if (new->flags & MPOL_F_REBINDING)
> -			mpol_rebind_policy(new, &mems, MPOL_REBIND_STEP2);
> -		else
> -			mpol_rebind_policy(new, &mems, MPOL_REBIND_ONCE);
> +		mpol_rebind_policy(new, &mems);
>  	}
>  	atomic_set(&new->refcnt, 1);
>  	return new;
> -- 
> 2.12.2

-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>