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Date: Mon, 24 Jun 2024 18:16:54 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Chris Li <chrisl@...nel.org>
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
Chris Li <chrisl@...nel.org> writes:
> 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?
You are right. This works.
However, I think that it's better to change as less as possible except
converting cluster list implementation. If we don't delete the cluster
from the free cluster list here and leave it to alloc_cluster(), does it
work? If so, we can make the patch smaller and focused. If necessary,
we can make the change in the later patch of the series.
>
>>
>> > + 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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