Re: [PATCH 1/2] mm: swap: swap cluster switch to double link list
From: Chris Li
Date: Tue May 28 2024 - 18:27:38 EST
Hi Kairui,
On Tue, May 28, 2024 at 9:24 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> On Sat, May 25, 2024 at 1:17 AM Chris Li <chrisl@xxxxxxxxxx> wrote:
> >
> > 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.
> > The assumption is that only the free cluster needs to be put
> > on the list.
> >
> > That assumption is not true for mTHP allocators any more. Need
> > to track the non full cluster on the list as well. Move the
> > current cluster single link list into standard double link list.
> >
> > Remove the cluster getter/setter for accessing the cluster
> > struct member. Move the cluster locking in the caller function
> > rather than the getter/setter function. That way the locking can
> > protect more than one member, e.g. cluster->flag.
> >
> > 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. Other than the list
> > conversion, there is no real function change in this patch.
> > ---
> > include/linux/swap.h | 14 ++--
> > mm/swapfile.c | 231 ++++++++++++++-------------------------------------
> > 2 files changed, 68 insertions(+), 177 deletions(-)
> >
>
> Hi Chris,
>
> Thanks for this very nice clean up, the code is much easier to read.
Thanks for the review.
See my comments below. I am working on a V2 to address the two issues
identified so far.
BTW, I am pretty happy the patch stats have much more deltes than insert.
>
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 11c53692f65f..0d3906eff3c9 100644
> > --- a/include/linux/swap.hm
> > +++ b/include/linux/swap.h
> > @@ -254,11 +254,12 @@ struct swap_cluster_info {
> > * elements correspond to the swap
> > * cluster
> > */
> > - unsigned int data:24;
> > + unsigned int count:16;
> > unsigned int flags:8;
> > + struct list_head next;
> > };
> > #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 +284,6 @@ 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 +296,7 @@ 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 */
> > @@ -333,7 +329,7 @@ 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 4f0e8b2ac8aa..205a60c5f9cb 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;
> > /*
> > * If scan_swap_map_slots() can't find a free cluster, it will check
> > * si->swap_map directly. To make sure the discarding cluster isn't
> > @@ -462,17 +355,16 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
> > memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> > SWAP_MAP_BAD, SWAPFILE_CLUSTER);
> >
> > - cluster_list_add_tail(&si->discard_clusters, si->cluster_info, idx);
> > -
> > + spin_lock_nested(&ci->lock, SINGLE_DEPTH_NESTING);
> > + list_add_tail(&ci->next, &si->discard_clusters);
> > + spin_unlock(&ci->lock);
> > schedule_work(&si->discard_work);
> > }
> >
> > -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;
> > -
> > - cluster_set_flag(ci + idx, CLUSTER_FLAG_FREE);
> > - cluster_list_add_tail(&si->free_clusters, ci, idx);
> > + ci->flags = CLUSTER_FLAG_FREE;
> > + list_add_tail(&ci->next, &si->free_clusters);
> > }
> >
> > /*
> > @@ -481,21 +373,21 @@ static void __free_cluster(struct swap_info_struct *si, unsigned long idx)
> > */
> > static void swap_do_scheduled_discard(struct swap_info_struct *si)
> > {
> > - struct swap_cluster_info *info, *ci;
> > + struct swap_cluster_info *ci;
> > unsigned int idx;
> >
> > - info = si->cluster_info;
> > -
> > - 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, next);
> > + 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);
> > + __free_cluster(si, ci);
> > memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> > 0, SWAPFILE_CLUSTER);
> > unlock_cluster(ci);
> > @@ -521,20 +413,20 @@ 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, next);
> >
> > - 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->next);
> > + 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 +434,11 @@ 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 +451,15 @@ 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 +473,20 @@ 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 +499,10 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
> > {
>
> This whole scan_swap_map_ssd_cluster_conflict function seems not
> needed now. free_clusters is a double linked list, so using a cluster
> in the middle won't corrupt the list. The comments are still for the
> old list design.
I was debating removing the cluster_conflict() as well and found out
it can't be removed until we change the order 0 allocations also use
clusters.
There can still be conflict because the order 0 allocations just do
the bruce force scan of swap_map[] when try_ssd fails. This causes
other problems as well. As far as I can tell, the conflict can still
happen.
>
> > struct percpu_cluster *percpu_cluster;
> > bool conflict;
> > -
> > + struct swap_cluster_info *first = list_first_entry(&si->free_clusters, struct swap_cluster_info, next);
> > 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 +543,14 @@ 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, next);
> > + list_del(&ci->next);
> > + spin_lock(&ci->lock);
>
> Shouldn't this list_del also be protected by ci->lock? It was
> protected in alloc_cluster before, keeping the flag synced with
> cluster status so cluster_is_free won't return false positive.
The list add and list del are protected by Si->lock not by cluster lock.
Previously I wanted to use cluster->lock to protect it and realized
that adding/deleting the cluster to/from the list will change three
clusters. (current, prev, next). We need to get three cluster locks.
We might change to a per list spinlock. e.g. one lock for one list to
reduce the contention on Si->lock. However, per cluster lock is not
enough if we only take one cluster lock.
>
> > + ci->flags = 0;
> > + 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
> > @@ -670,7 +562,8 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
> > goto new_cluster;
> > } else
> > return false;
> > - }
> > + } else
> > + ci = si->cluster_info + tmp;
>
> This "else ci = ..." seems wrong, tmp is not an array index, and not
> needed either.
Yes, there is a bug there, pointed out by OPPO as well. It should be
ci = si->cluster_info + (tmp/ SWAPFILE_CLUSTER);
"tmp" is needed because "tmp" or " cluster->next[order]" keep track of
the current cluster allocation offset,
in the per cpu cluster struct.
BTW, In my V2 I have changed "tmp" to "offset" and previous "offset"
to "retoffset" to make it more obvious. "tmp" does not give much
information about what it really does.
Chris
>
> >
> > /*
> > * Other CPUs can use our cluster if they can't find a free cluster,
> > @@ -1062,8 +955,9 @@ static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
> >
> > ci = lock_cluster(si, offset);
> > memset(si->swap_map + offset, 0, SWAPFILE_CLUSTER);
> > - cluster_set_count_flag(ci, 0, 0);
> > - free_cluster(si, idx);
> > + ci->count = 0;
> > + ci->flags = 0;
> > + free_cluster(si, ci);
> > unlock_cluster(ci);
> > swap_range_free(si, offset, SWAPFILE_CLUSTER);
> > }
> > @@ -1336,7 +1230,7 @@ static void swap_entry_free(struct swap_info_struct *p, swp_entry_t entry)
> > count = p->swap_map[offset];
> > VM_BUG_ON(count != SWAP_HAS_CACHE);
> > p->swap_map[offset] = 0;
> > - dec_cluster_info_page(p, p->cluster_info, offset);
> > + dec_cluster_info_page(p, ci);
> > unlock_cluster(ci);
> >
> > mem_cgroup_uncharge_swap(entry, 1);
> > @@ -2985,8 +2879,8 @@ static int setup_swap_map_and_extents(struct swap_info_struct *p,
> >
> > nr_good_pages = maxpages - 1; /* omit header page */
> >
> > - cluster_list_init(&p->free_clusters);
> > - cluster_list_init(&p->discard_clusters);
> > + INIT_LIST_HEAD(&p->free_clusters);
> > + INIT_LIST_HEAD(&p->discard_clusters);
> >
> > for (i = 0; i < swap_header->info.nr_badpages; i++) {
> > unsigned int page_nr = swap_header->info.badpages[i];
> > @@ -3037,14 +2931,15 @@ static int setup_swap_map_and_extents(struct swap_info_struct *p,
> > for (k = 0; k < SWAP_CLUSTER_COLS; k++) {
> > j = (k + col) % SWAP_CLUSTER_COLS;
> > for (i = 0; i < DIV_ROUND_UP(nr_clusters, SWAP_CLUSTER_COLS); i++) {
> > + struct swap_cluster_info *ci;
> > idx = i * SWAP_CLUSTER_COLS + j;
> > + ci = cluster_info + idx;
> > if (idx >= nr_clusters)
> > continue;
> > - if (cluster_count(&cluster_info[idx]))
> > + if (ci->count)
> > continue;
> > - cluster_set_flag(&cluster_info[idx], CLUSTER_FLAG_FREE);
> > - cluster_list_add_tail(&p->free_clusters, cluster_info,
> > - idx);
> > + ci->flags = CLUSTER_FLAG_FREE;
> > + list_add_tail(&ci->next, &p->free_clusters);
> > }
> > }
> > return nr_extents;
> >
> > --
> > 2.45.1.288.g0e0cd299f1-goog
> >
> >
>