Re: [PATCH v3 08/13] mm, swap: use an enum to define all cluster flags and wrap flags changes

[Baoquan He <bhe@redhat.com>]

On 12/31/24 at 01:46am, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> Currently, we are only using flags to indicate which list the cluster
> is on. Using one bit for each list type might be a waste, as the list
> type grows, we will consume too many bits. Additionally, the current
> mixed usage of '&' and '==' is a bit confusing.

I think this kind of converting can only happen when the type is
exclusive on each cluster. Then we can set and use
'ci->flags == CLUSTER_FLAG_XXX' to check it.

> 
> Make it clean by using an enum to define all possible cluster
> statuses. Only an off-list cluster will have the NONE (0) flag.
> And use a wrapper to annotate and sanitize all flag settings
> and list movements.
> 
> Suggested-by: Chris Li <chrisl@kernel.org>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  include/linux/swap.h | 17 +++++++---
>  mm/swapfile.c        | 75 +++++++++++++++++++++++---------------------
>  2 files changed, 52 insertions(+), 40 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 02120f1005d5..339d7f0192ff 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -257,10 +257,19 @@ struct swap_cluster_info {
>  	u8 order;
>  	struct list_head list;
>  };
> -#define CLUSTER_FLAG_FREE 1 /* This cluster is free */
> -#define CLUSTER_FLAG_NONFULL 2 /* This cluster is on nonfull list */
> -#define CLUSTER_FLAG_FRAG 4 /* This cluster is on nonfull list */
> -#define CLUSTER_FLAG_FULL 8 /* This cluster is on full list */
> +
> +/* All on-list cluster must have a non-zero flag. */
> +enum swap_cluster_flags {
> +	CLUSTER_FLAG_NONE = 0, /* For temporary off-list cluster */
> +	CLUSTER_FLAG_FREE,
> +	CLUSTER_FLAG_NONFULL,
> +	CLUSTER_FLAG_FRAG,
> +	/* Clusters with flags above are allocatable */
> +	CLUSTER_FLAG_USABLE = CLUSTER_FLAG_FRAG,
> +	CLUSTER_FLAG_FULL,
> +	CLUSTER_FLAG_DISCARD,
> +	CLUSTER_FLAG_MAX,
> +};
>  
>  /*
>   * The first page in the swap file is the swap header, which is always marked
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 99fd0b0d84a2..7795a3d27273 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -403,7 +403,7 @@ static void discard_swap_cluster(struct swap_info_struct *si,
>  
>  static inline bool cluster_is_free(struct swap_cluster_info *info)
>  {
> -	return info->flags & CLUSTER_FLAG_FREE;
> +	return info->flags == CLUSTER_FLAG_FREE;
>  }
>  
>  static inline unsigned int cluster_index(struct swap_info_struct *si,
> @@ -434,6 +434,27 @@ static inline void unlock_cluster(struct swap_cluster_info *ci)
>  	spin_unlock(&ci->lock);
>  }
>  
> +static void cluster_move(struct swap_info_struct *si,
               ~~~~~~~~~~~~
Maybe rename it to move_cluster() which has the same naming style as
lock_cluster()/unlock_cluster()? This is what we usually do namin if a
function is action acts on objects.

Other than this, this patch looks great to me.

> +			 struct swap_cluster_info *ci, struct list_head *list,
> +			 enum swap_cluster_flags new_flags)
> +{
> +	VM_WARN_ON(ci->flags == new_flags);
> +	BUILD_BUG_ON(1 << sizeof(ci->flags) * BITS_PER_BYTE < CLUSTER_FLAG_MAX);
> +
> +	if (ci->flags == CLUSTER_FLAG_NONE) {
> +		list_add_tail(&ci->list, list);
> +	} else {
> +		if (ci->flags == CLUSTER_FLAG_FRAG) {
> +			VM_WARN_ON(!si->frag_cluster_nr[ci->order]);
> +			si->frag_cluster_nr[ci->order]--;
> +		}
> +		list_move_tail(&ci->list, list);
> +	}
> +	ci->flags = new_flags;
> +	if (new_flags == CLUSTER_FLAG_FRAG)
> +		si->frag_cluster_nr[ci->order]++;
> +}
> +
>  /* Add a cluster to discard list and schedule it to do discard */
>  static void swap_cluster_schedule_discard(struct swap_info_struct *si,
>  		struct swap_cluster_info *ci)
> @@ -447,10 +468,8 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
>  	 */
>  	memset(si->swap_map + idx * SWAPFILE_CLUSTER,
>  			SWAP_MAP_BAD, SWAPFILE_CLUSTER);
> -
> -	VM_BUG_ON(ci->flags & CLUSTER_FLAG_FREE);
> -	list_move_tail(&ci->list, &si->discard_clusters);
> -	ci->flags = 0;
> +	VM_BUG_ON(ci->flags == CLUSTER_FLAG_FREE);
> +	cluster_move(si, ci, &si->discard_clusters, CLUSTER_FLAG_DISCARD);
>  	schedule_work(&si->discard_work);
>  }
>  
> @@ -458,12 +477,7 @@ static void __free_cluster(struct swap_info_struct *si, struct swap_cluster_info
>  {
>  	lockdep_assert_held(&si->lock);
>  	lockdep_assert_held(&ci->lock);
> -
> -	if (ci->flags)
> -		list_move_tail(&ci->list, &si->free_clusters);
> -	else
> -		list_add_tail(&ci->list, &si->free_clusters);
> -	ci->flags = CLUSTER_FLAG_FREE;
> +	cluster_move(si, ci, &si->free_clusters, CLUSTER_FLAG_FREE);
>  	ci->order = 0;
>  }
>  
> @@ -479,6 +493,8 @@ static void swap_do_scheduled_discard(struct swap_info_struct *si)
>  	while (!list_empty(&si->discard_clusters)) {
>  		ci = list_first_entry(&si->discard_clusters, struct swap_cluster_info, list);
>  		list_del(&ci->list);
> +		/* Must clear flag when taking a cluster off-list */
> +		ci->flags = CLUSTER_FLAG_NONE;
>  		idx = cluster_index(si, ci);
>  		spin_unlock(&si->lock);
>  
> @@ -519,9 +535,6 @@ static void free_cluster(struct swap_info_struct *si, struct swap_cluster_info *
>  	lockdep_assert_held(&si->lock);
>  	lockdep_assert_held(&ci->lock);
>  
> -	if (ci->flags & CLUSTER_FLAG_FRAG)
> -		si->frag_cluster_nr[ci->order]--;
> -
>  	/*
>  	 * If the swap is discardable, prepare discard the cluster
>  	 * instead of free it immediately. The cluster will be freed
> @@ -573,13 +586,9 @@ static void dec_cluster_info_page(struct swap_info_struct *si,
>  		return;
>  	}
>  
> -	if (!(ci->flags & CLUSTER_FLAG_NONFULL)) {
> -		VM_BUG_ON(ci->flags & CLUSTER_FLAG_FREE);
> -		if (ci->flags & CLUSTER_FLAG_FRAG)
> -			si->frag_cluster_nr[ci->order]--;
> -		list_move_tail(&ci->list, &si->nonfull_clusters[ci->order]);
> -		ci->flags = CLUSTER_FLAG_NONFULL;
> -	}
> +	if (ci->flags != CLUSTER_FLAG_NONFULL)
> +		cluster_move(si, ci, &si->nonfull_clusters[ci->order],
> +			     CLUSTER_FLAG_NONFULL);
>  }
>  
>  static bool cluster_reclaim_range(struct swap_info_struct *si,
> @@ -663,11 +672,13 @@ static bool cluster_alloc_range(struct swap_info_struct *si, struct swap_cluster
>  	if (!(si->flags & SWP_WRITEOK))
>  		return false;
>  
> +	VM_BUG_ON(ci->flags == CLUSTER_FLAG_NONE);
> +	VM_BUG_ON(ci->flags > CLUSTER_FLAG_USABLE);
> +
>  	if (cluster_is_free(ci)) {
> -		if (nr_pages < SWAPFILE_CLUSTER) {
> -			list_move_tail(&ci->list, &si->nonfull_clusters[order]);
> -			ci->flags = CLUSTER_FLAG_NONFULL;
> -		}
> +		if (nr_pages < SWAPFILE_CLUSTER)
> +			cluster_move(si, ci, &si->nonfull_clusters[order],
> +				     CLUSTER_FLAG_NONFULL);
>  		ci->order = order;
>  	}
>  
> @@ -675,14 +686,8 @@ static bool cluster_alloc_range(struct swap_info_struct *si, struct swap_cluster
>  	swap_range_alloc(si, nr_pages);
>  	ci->count += nr_pages;
>  
> -	if (ci->count == SWAPFILE_CLUSTER) {
> -		VM_BUG_ON(!(ci->flags &
> -			  (CLUSTER_FLAG_FREE | CLUSTER_FLAG_NONFULL | CLUSTER_FLAG_FRAG)));
> -		if (ci->flags & CLUSTER_FLAG_FRAG)
> -			si->frag_cluster_nr[ci->order]--;
> -		list_move_tail(&ci->list, &si->full_clusters);
> -		ci->flags = CLUSTER_FLAG_FULL;
> -	}
> +	if (ci->count == SWAPFILE_CLUSTER)
> +		cluster_move(si, ci, &si->full_clusters, CLUSTER_FLAG_FULL);
>  
>  	return true;
>  }
> @@ -821,9 +826,7 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
>  		while (!list_empty(&si->nonfull_clusters[order])) {
>  			ci = list_first_entry(&si->nonfull_clusters[order],
>  					      struct swap_cluster_info, list);
> -			list_move_tail(&ci->list, &si->frag_clusters[order]);
> -			ci->flags = CLUSTER_FLAG_FRAG;
> -			si->frag_cluster_nr[order]++;
> +			cluster_move(si, ci, &si->frag_clusters[order], CLUSTER_FLAG_FRAG);
>  			offset = alloc_swap_scan_cluster(si, cluster_offset(si, ci),
>  							 &found, order, usage);
>  			frags++;
> -- 
> 2.47.1
> 
>