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Date: Fri, 21 Jun 2024 11:55:23 -0700
From: Chris Li <chrisl@...nel.org>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Kairui Song <kasong@...cent.com>,
Ryan Roberts <ryan.roberts@....com>, Kalesh Singh <kaleshsingh@...gle.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Barry Song <baohua@...nel.org>
Subject: Re: [PATCH v3 1/2] mm: swap: swap cluster switch to double link list
On Thu, Jun 20, 2024 at 8:01 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Chris Li <chrisl@...nel.org> writes:
>
> > Previously, the swap cluster used a cluster index as a pointer
> > to construct a custom single link list type "swap_cluster_list".
> > The next cluster pointer is shared with the cluster->count.
> > It prevents puting the non free cluster into a list.
> >
> > Change the cluster to use the standard double link list instead.
> > This allows tracing the nonfull cluster in the follow up patch.
> > That way, it is faster to get to the nonfull cluster of that order.
> >
> > Remove the cluster getter/setter for accessing the cluster
> > struct member.
> >
> > The list operation is protected by the swap_info_struct->lock.
> >
> > Change cluster code to use "struct swap_cluster_info *" to
> > reference the cluster rather than by using index. That is more
> > consistent with the list manipulation. It avoids the repeat
> > adding index to the cluser_info. The code is easier to understand.
> >
> > Remove the cluster next pointer is NULL flag, the double link
> > list can handle the empty list pretty well.
> >
> > The "swap_cluster_info" struct is two pointer bigger, because
> > 512 swap entries share one swap struct, it has very little impact
> > on the average memory usage per swap entry. For 1TB swapfile, the
> > swap cluster data structure increases from 8MB to 24MB.
> >
> > Other than the list conversion, there is no real function change
> > in this patch.
> >
> > Signed-off-by: Chris Li <chrisl@...nel.org>
> > ---
> > include/linux/swap.h | 26 +++---
> > mm/swapfile.c | 227 ++++++++++++++-------------------------------------
> > 2 files changed, 70 insertions(+), 183 deletions(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 3df75d62a835..690a04f06674 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -243,22 +243,21 @@ enum {
> > * free clusters are organized into a list. We fetch an entry from the list to
> > * get a free cluster.
> > *
> > - * The data field stores next cluster if the cluster is free or cluster usage
> > - * counter otherwise. The flags field determines if a cluster is free. This is
> > - * protected by swap_info_struct.lock.
> > + * The flags field determines if a cluster is free. This is
> > + * protected by cluster lock.
> > */
> > struct swap_cluster_info {
> > spinlock_t lock; /*
> > * Protect swap_cluster_info fields
> > - * and swap_info_struct->swap_map
> > - * elements correspond to the swap
> > - * cluster
> > + * other than list, and swap_info_struct->swap_map
> > + * elements correspond to the swap cluster.
> > */
> > - unsigned int data:24;
> > - unsigned int flags:8;
> > + u16 count;
> > + u8 flags;
> > + struct list_head list;
> > };
> > #define CLUSTER_FLAG_FREE 1 /* This cluster is free */
> > -#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
> > +
> >
> > /*
> > * The first page in the swap file is the swap header, which is always marked
> > @@ -283,11 +282,6 @@ struct percpu_cluster {
> > unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
> > };
> >
> > -struct swap_cluster_list {
> > - struct swap_cluster_info head;
> > - struct swap_cluster_info tail;
> > -};
> > -
> > /*
> > * The in-memory structure used to track swap areas.
> > */
> > @@ -300,7 +294,7 @@ struct swap_info_struct {
> > unsigned int max; /* extent of the swap_map */
> > unsigned char *swap_map; /* vmalloc'ed array of usage counts */
> > struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
> > - struct swap_cluster_list free_clusters; /* free clusters list */
> > + struct list_head free_clusters; /* free clusters list */
> > unsigned int lowest_bit; /* index of first free in swap_map */
> > unsigned int highest_bit; /* index of last free in swap_map */
> > unsigned int pages; /* total of usable pages of swap */
> > @@ -331,7 +325,7 @@ struct swap_info_struct {
> > * list.
> > */
> > struct work_struct discard_work; /* discard worker */
> > - struct swap_cluster_list discard_clusters; /* discard clusters list */
> > + struct list_head discard_clusters; /* discard clusters list */
> > struct plist_node avail_lists[]; /*
> > * entries in swap_avail_heads, one
> > * entry per node.
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 9c6d8e557c0f..0b11c437f9cc 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -290,64 +290,11 @@ static void discard_swap_cluster(struct swap_info_struct *si,
> > #endif
> > #define LATENCY_LIMIT 256
> >
> > -static inline void cluster_set_flag(struct swap_cluster_info *info,
> > - unsigned int flag)
> > -{
> > - info->flags = flag;
> > -}
> > -
> > -static inline unsigned int cluster_count(struct swap_cluster_info *info)
> > -{
> > - return info->data;
> > -}
> > -
> > -static inline void cluster_set_count(struct swap_cluster_info *info,
> > - unsigned int c)
> > -{
> > - info->data = c;
> > -}
> > -
> > -static inline void cluster_set_count_flag(struct swap_cluster_info *info,
> > - unsigned int c, unsigned int f)
> > -{
> > - info->flags = f;
> > - info->data = c;
> > -}
> > -
> > -static inline unsigned int cluster_next(struct swap_cluster_info *info)
> > -{
> > - return info->data;
> > -}
> > -
> > -static inline void cluster_set_next(struct swap_cluster_info *info,
> > - unsigned int n)
> > -{
> > - info->data = n;
> > -}
> > -
> > -static inline void cluster_set_next_flag(struct swap_cluster_info *info,
> > - unsigned int n, unsigned int f)
> > -{
> > - info->flags = f;
> > - info->data = n;
> > -}
> > -
> > static inline bool cluster_is_free(struct swap_cluster_info *info)
> > {
> > return info->flags & CLUSTER_FLAG_FREE;
> > }
> >
> > -static inline bool cluster_is_null(struct swap_cluster_info *info)
> > -{
> > - return info->flags & CLUSTER_FLAG_NEXT_NULL;
> > -}
> > -
> > -static inline void cluster_set_null(struct swap_cluster_info *info)
> > -{
> > - info->flags = CLUSTER_FLAG_NEXT_NULL;
> > - info->data = 0;
> > -}
> > -
> > static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
> > unsigned long offset)
> > {
> > @@ -394,65 +341,11 @@ static inline void unlock_cluster_or_swap_info(struct swap_info_struct *si,
> > spin_unlock(&si->lock);
> > }
> >
> > -static inline bool cluster_list_empty(struct swap_cluster_list *list)
> > -{
> > - return cluster_is_null(&list->head);
> > -}
> > -
> > -static inline unsigned int cluster_list_first(struct swap_cluster_list *list)
> > -{
> > - return cluster_next(&list->head);
> > -}
> > -
> > -static void cluster_list_init(struct swap_cluster_list *list)
> > -{
> > - cluster_set_null(&list->head);
> > - cluster_set_null(&list->tail);
> > -}
> > -
> > -static void cluster_list_add_tail(struct swap_cluster_list *list,
> > - struct swap_cluster_info *ci,
> > - unsigned int idx)
> > -{
> > - if (cluster_list_empty(list)) {
> > - cluster_set_next_flag(&list->head, idx, 0);
> > - cluster_set_next_flag(&list->tail, idx, 0);
> > - } else {
> > - struct swap_cluster_info *ci_tail;
> > - unsigned int tail = cluster_next(&list->tail);
> > -
> > - /*
> > - * Nested cluster lock, but both cluster locks are
> > - * only acquired when we held swap_info_struct->lock
> > - */
> > - ci_tail = ci + tail;
> > - spin_lock_nested(&ci_tail->lock, SINGLE_DEPTH_NESTING);
> > - cluster_set_next(ci_tail, idx);
> > - spin_unlock(&ci_tail->lock);
> > - cluster_set_next_flag(&list->tail, idx, 0);
> > - }
> > -}
> > -
> > -static unsigned int cluster_list_del_first(struct swap_cluster_list *list,
> > - struct swap_cluster_info *ci)
> > -{
> > - unsigned int idx;
> > -
> > - idx = cluster_next(&list->head);
> > - if (cluster_next(&list->tail) == idx) {
> > - cluster_set_null(&list->head);
> > - cluster_set_null(&list->tail);
> > - } else
> > - cluster_set_next_flag(&list->head,
> > - cluster_next(&ci[idx]), 0);
> > -
> > - return idx;
> > -}
> > -
> > /* Add a cluster to discard list and schedule it to do discard */
> > static void swap_cluster_schedule_discard(struct swap_info_struct *si,
> > - unsigned int idx)
> > + struct swap_cluster_info *ci)
> > {
> > + unsigned int idx = ci - si->cluster_info;
>
> IIRC, we have reached consensus that we will add a helper for this
> pattern.
Yes, sorry about that. When I drop the V2 fixup branch, I forget to update that.
>
> > - while (!cluster_list_empty(&si->discard_clusters)) {
> > - idx = cluster_list_del_first(&si->discard_clusters, info);
> > + while (!list_empty(&si->discard_clusters)) {
> > + ci = list_first_entry(&si->discard_clusters, struct swap_cluster_info, list);
> > + list_del(&ci->list);
> > + idx = ci - si->cluster_info;
> > spin_unlock(&si->lock);
> >
> > discard_swap_cluster(si, idx * SWAPFILE_CLUSTER,
> > SWAPFILE_CLUSTER);
> >
> > spin_lock(&si->lock);
> > - ci = lock_cluster(si, idx * SWAPFILE_CLUSTER);
> > - __free_cluster(si, idx);
> > +
> > + spin_lock(&ci->lock);
>
> If you really don't like lock_cluster(), please replace unlock_cluster()
> below with spin_unlock() too.
>
Done. Switch to spin_unlock().
> > + __free_cluster(si, ci);
> > memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> > 0, SWAPFILE_CLUSTER);
> > unlock_cluster(ci);
> > @@ -521,20 +412,20 @@ static void swap_users_ref_free(struct percpu_ref *ref)
> > complete(&si->comp);
> > }
> >
> > -static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
> > +static struct swap_cluster_info *alloc_cluster(struct swap_info_struct *si, unsigned long idx)
> > {
> > - struct swap_cluster_info *ci = si->cluster_info;
> > + struct swap_cluster_info *ci = list_first_entry(&si->free_clusters, struct swap_cluster_info, list);
> >
> > - VM_BUG_ON(cluster_list_first(&si->free_clusters) != idx);
> > - cluster_list_del_first(&si->free_clusters, ci);
> > - cluster_set_count_flag(ci + idx, 0, 0);
> > + VM_BUG_ON(ci - si->cluster_info != idx);
> > + list_del(&ci->list);
> > + ci->count = 0;
> > + ci->flags = 0;
> > + return ci;
> > }
> >
> > -static void free_cluster(struct swap_info_struct *si, unsigned long idx)
> > +static void free_cluster(struct swap_info_struct *si, struct swap_cluster_info *ci)
> > {
> > - struct swap_cluster_info *ci = si->cluster_info + idx;
> > -
> > - VM_BUG_ON(cluster_count(ci) != 0);
> > + VM_BUG_ON(ci->count != 0);
> > /*
> > * If the swap is discardable, prepare discard the cluster
> > * instead of free it immediately. The cluster will be freed
> > @@ -542,11 +433,11 @@ static void free_cluster(struct swap_info_struct *si, unsigned long idx)
> > */
> > if ((si->flags & (SWP_WRITEOK | SWP_PAGE_DISCARD)) ==
> > (SWP_WRITEOK | SWP_PAGE_DISCARD)) {
> > - swap_cluster_schedule_discard(si, idx);
> > + swap_cluster_schedule_discard(si, ci);
> > return;
> > }
> >
> > - __free_cluster(si, idx);
> > + __free_cluster(si, ci);
> > }
> >
> > /*
> > @@ -559,15 +450,15 @@ static void add_cluster_info_page(struct swap_info_struct *p,
> > unsigned long count)
> > {
> > unsigned long idx = page_nr / SWAPFILE_CLUSTER;
> > + struct swap_cluster_info *ci = cluster_info + idx;
> >
> > if (!cluster_info)
> > return;
> > - if (cluster_is_free(&cluster_info[idx]))
> > + if (cluster_is_free(ci))
> > alloc_cluster(p, idx);
> >
> > - VM_BUG_ON(cluster_count(&cluster_info[idx]) + count > SWAPFILE_CLUSTER);
> > - cluster_set_count(&cluster_info[idx],
> > - cluster_count(&cluster_info[idx]) + count);
> > + VM_BUG_ON(ci->count + count > SWAPFILE_CLUSTER);
> > + ci->count += count;
> > }
> >
> > /*
> > @@ -581,24 +472,20 @@ static void inc_cluster_info_page(struct swap_info_struct *p,
> > }
> >
> > /*
> > - * The cluster corresponding to page_nr decreases one usage. If the usage
> > - * counter becomes 0, which means no page in the cluster is in using, we can
> > - * optionally discard the cluster and add it to free cluster list.
> > + * The cluster ci decreases one usage. If the usage counter becomes 0,
> > + * which means no page in the cluster is in using, we can optionally discard
> > + * the cluster and add it to free cluster list.
> > */
> > -static void dec_cluster_info_page(struct swap_info_struct *p,
> > - struct swap_cluster_info *cluster_info, unsigned long page_nr)
> > +static void dec_cluster_info_page(struct swap_info_struct *p, struct swap_cluster_info *ci)
> > {
> > - unsigned long idx = page_nr / SWAPFILE_CLUSTER;
> > -
> > - if (!cluster_info)
> > + if (!p->cluster_info)
> > return;
> >
> > - VM_BUG_ON(cluster_count(&cluster_info[idx]) == 0);
> > - cluster_set_count(&cluster_info[idx],
> > - cluster_count(&cluster_info[idx]) - 1);
> > + VM_BUG_ON(ci->count == 0);
> > + ci->count--;
> >
> > - if (cluster_count(&cluster_info[idx]) == 0)
> > - free_cluster(p, idx);
> > + if (!ci->count)
> > + free_cluster(p, ci);
> > }
> >
> > /*
> > @@ -611,10 +498,10 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
> > {
> > struct percpu_cluster *percpu_cluster;
> > bool conflict;
> > -
> > + struct swap_cluster_info *first = list_first_entry(&si->free_clusters, struct swap_cluster_info, list);
> > offset /= SWAPFILE_CLUSTER;
> > - conflict = !cluster_list_empty(&si->free_clusters) &&
> > - offset != cluster_list_first(&si->free_clusters) &&
> > + conflict = !list_empty(&si->free_clusters) &&
> > + offset != first - si->cluster_info &&
> > cluster_is_free(&si->cluster_info[offset]);
> >
> > if (!conflict)
> > @@ -655,10 +542,14 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
> > cluster = this_cpu_ptr(si->percpu_cluster);
> > tmp = cluster->next[order];
> > if (tmp == SWAP_NEXT_INVALID) {
> > - if (!cluster_list_empty(&si->free_clusters)) {
> > - tmp = cluster_next(&si->free_clusters.head) *
> > - SWAPFILE_CLUSTER;
> > - } else if (!cluster_list_empty(&si->discard_clusters)) {
> > + if (!list_empty(&si->free_clusters)) {
> > + ci = list_first_entry(&si->free_clusters, struct swap_cluster_info, list);
> > + list_del(&ci->list);
>
> IIRC, we shouldn't delete the cluster from the free list here, because
> we will delete it later in alloc_cluster().
>
> > + spin_lock(&ci->lock);
> > + ci->flags = 0;
>
> And also shouldn't set ci->flags here too. Because the cluster may be
> kept in free list if scan_swap_map_ssd_cluster_conflict() returns true.
If we both delete the cluster from the free list and clear the flag
here. alloc_cluster() will not see this cluster because the flags have
been cleared.
Will that work?
Chris
>
> > + spin_unlock(&ci->lock);
> > + tmp = (ci - si->cluster_info) * SWAPFILE_CLUSTER;
> > + } else if (!list_empty(&si->discard_clusters)) {
> > /*
> > * we don't have free cluster but have some clusters in
> > * discarding, do discard now and reclaim them, then
>
> [snip]
>
> --
> Best Regards,
> Huang, Ying
>
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