Re: [PATCH v3 09/13] mm, swap: reduce contention on device lock

Kairui Song
2025-01-09T02:15:29Z

On Wed, Jan 8, 2025 at 7:10 PM Baoquan He <bhe@redhat.com> wrote:
>

Thanks for the very detailed review!

> On 12/31/24 at 01:46am, Kairui Song wrote:
> ......snip.....
> > ---
> > include/linux/swap.h | 3 +-
> > mm/swapfile.c | 435 ++++++++++++++++++++++++-------------------
> > 2 files changed, 246 insertions(+), 192 deletions(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 339d7f0192ff..c4ff31cb6bde 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -291,6 +291,7 @@ enum swap_cluster_flags {
> > * throughput.
> > */
> > struct percpu_cluster {
> > + local_lock_t lock; /* Protect the percpu_cluster above */
> > unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
> > };
> >
> > @@ -313,7 +314,7 @@ struct swap_info_struct {
> > /* list of cluster that contains at least one free slot */
> > struct list_head frag_clusters[SWAP_NR_ORDERS];
> > /* list of cluster that are fragmented or contented */
> > - unsigned int frag_cluster_nr[SWAP_NR_ORDERS];
> > + atomic_long_t frag_cluster_nr[SWAP_NR_ORDERS];
> > unsigned int pages; /* total of usable pages of swap */
> > atomic_long_t inuse_pages; /* number of those currently in use */
> > struct percpu_cluster __percpu *percpu_cluster; /* per cpu's swap location */
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 7795a3d27273..dadd4fead689 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -261,12 +261,10 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> > folio_ref_sub(folio, nr_pages);
> > folio_set_dirty(folio);
> >
> > - spin_lock(&si->lock);
> > /* Only sinple page folio can be backed by zswap */
> > if (nr_pages == 1)
> > zswap_invalidate(entry);
> > swap_entry_range_free(si, entry, nr_pages);
> > - spin_unlock(&si->lock);
> > ret = nr_pages;
> > out_unlock:
> > folio_unlock(folio);
> > @@ -403,7 +401,21 @@ 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->count == 0;
>
> This is a little confusing. Maybe we should add one and call it
> cluster_is_empty(). Because discarded clusters are also be able to pass
> the checking here.

Good idea, agree on this, this new name is better.

>
> > +}
> > +
> > +static inline bool cluster_is_discard(struct swap_cluster_info *info)
> > +{
> > + return info->flags == CLUSTER_FLAG_DISCARD;
> > +}
> > +
> > +static inline bool cluster_is_usable(struct swap_cluster_info *ci, int order)
> > +{
> > + if (unlikely(ci->flags > CLUSTER_FLAG_USABLE))
> > + return false;
> > + if (!order)
> > + return true;
> > + return cluster_is_free(ci) || order == ci->order;
> > }
> >
> > static inline unsigned int cluster_index(struct swap_info_struct *si,
> > @@ -440,19 +452,20 @@ static void cluster_move(struct swap_info_struct *si,
> > {
> > VM_WARN_ON(ci->flags == new_flags);
> > BUILD_BUG_ON(1 << sizeof(ci->flags) * BITS_PER_BYTE < CLUSTER_FLAG_MAX);
> > + lockdep_assert_held(&ci->lock);
> >
> > - if (ci->flags == CLUSTER_FLAG_NONE) {
> > + spin_lock(&si->lock);
> > + 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]--;
> > - }
> > + else
> > list_move_tail(&ci->list, list);
> > - }
> > + spin_unlock(&si->lock);
> > +
> > + if (ci->flags == CLUSTER_FLAG_FRAG)
> > + atomic_long_dec(&si->frag_cluster_nr[ci->order]);
> > + else if (new_flags == CLUSTER_FLAG_FRAG)
> > + atomic_long_inc(&si->frag_cluster_nr[ci->order]);
> > 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 */
> > @@ -475,39 +488,90 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
> >
> > static void __free_cluster(struct swap_info_struct *si, struct swap_cluster_info *ci)
> > {
> > - lockdep_assert_held(&si->lock);
> > lockdep_assert_held(&ci->lock);
> > cluster_move(si, ci, &si->free_clusters, CLUSTER_FLAG_FREE);
> > ci->order = 0;
> > }
> >
> > +/*
> > + * Isolate and lock the first cluster that is not contented on a list,
> > + * clean its flag before taken off-list. Cluster flag must be in sync
> > + * with list status, so cluster updaters can always know the cluster
> > + * list status without touching si lock.
> > + *
> > + * Note it's possible that all clusters on a list are contented so
> > + * this returns NULL for an non-empty list.
> > + */
> > +static struct swap_cluster_info *cluster_isolate_lock(
> > + struct swap_info_struct *si, struct list_head *list)
> > +{
> > + struct swap_cluster_info *ci, *ret = NULL;
> > +
> > + spin_lock(&si->lock);
> > +
> > + if (unlikely(!(si->flags & SWP_WRITEOK)))
> > + goto out;
> > +
> > + list_for_each_entry(ci, list, list) {
> > + if (!spin_trylock(&ci->lock))
> > + continue;
> > +
> > + /* We may only isolate and clear flags of following lists */
> > + VM_BUG_ON(!ci->flags);
> > + VM_BUG_ON(ci->flags > CLUSTER_FLAG_USABLE &&
> > + ci->flags != CLUSTER_FLAG_FULL);
> > +
> > + list_del(&ci->list);
> > + ci->flags = CLUSTER_FLAG_NONE;
> > + ret = ci;
> > + break;
> > + }
> > +out:
> > + spin_unlock(&si->lock);
> > +
> > + return ret;
> > +}
> > +
> > /*
> > * Doing discard actually. After a cluster discard is finished, the cluster
> > - * will be added to free cluster list. caller should hold si->lock.
> > -*/
> > -static void swap_do_scheduled_discard(struct swap_info_struct *si)
> > + * will be added to free cluster list. Discard cluster is a bit special as
> > + * they don't participate in allocation or reclaim, so clusters marked as
> > + * CLUSTER_FLAG_DISCARD must remain off-list or on discard list.
> > + */
> > +static bool swap_do_scheduled_discard(struct swap_info_struct *si)
> > {
> > struct swap_cluster_info *ci;
> > + bool ret = false;
> > unsigned int idx;
> >
> > + spin_lock(&si->lock);
> > while (!list_empty(&si->discard_clusters)) {
> > ci = list_first_entry(&si->discard_clusters, struct swap_cluster_info, list);
> > + /*
> > + * Delete the cluster from list but don't clear its flags until
> > + * discard is done, so isolation and relocation will skip it.
> > + */
> > list_del(&ci->list);
>
> I don't understand above comment. ci has been taken off list. While
> allocation need isolate from a usable list. Even though we clear
> ci->flags now, how come isolation and relocation will touch it. I may
> miss anything here.

There are many cases, one possible and common situation is that the
percpu cluster (si->percpu_cluster of another CPU) is still pointing
to it.

Also, this commit removed protection of si lock on allocation, and
allocation path may also drop ci lock to call reclaim, which means one
cluster could be used or freed by anyone before allocator reacquire
the ci lock again. In that case, the allocator could see a discard
cluster.

So we don't clear the discard flag, in case anyone misuse it.

I can add more inline comments on this, this is already some related
comments above the function relocate_cluster, could add some more
referencing that.

>
> > - /* Must clear flag when taking a cluster off-list */
> > - ci->flags = CLUSTER_FLAG_NONE;
> > idx = cluster_index(si, ci);
> > spin_unlock(&si->lock);
> > -
> > discard_swap_cluster(si, idx * SWAPFILE_CLUSTER,
> > SWAPFILE_CLUSTER);
> >
> > - spin_lock(&si->lock);
> > spin_lock(&ci->lock);
> > - __free_cluster(si, ci);
> > + /*
> > + * Discard is done, clear its flags as it's now off-list,
> > + * then return the cluster to allocation list.
> > + */
> > + ci->flags = CLUSTER_FLAG_NONE;
> > memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> > 0, SWAPFILE_CLUSTER);
> > + __free_cluster(si, ci);
> > spin_unlock(&ci->lock);
> > + ret = true;
> > + spin_lock(&si->lock);
> > }
> > + spin_unlock(&si->lock);
> > + return ret;
> > }
> >
> > static void swap_discard_work(struct work_struct *work)
> ......snip....
> > @@ -791,29 +873,34 @@ static void swap_reclaim_work(struct work_struct *work)
> > static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int order,
> > unsigned char usage)
> > {
> > - struct percpu_cluster *cluster;
> > struct swap_cluster_info *ci;
> > unsigned int offset, found = 0;
> >
> > -new_cluster:
> > - lockdep_assert_held(&si->lock);
> > - cluster = this_cpu_ptr(si->percpu_cluster);
> > - offset = cluster->next[order];
> > + /* Fast path using per CPU cluster */
> > + local_lock(&si->percpu_cluster->lock);
> > + offset = __this_cpu_read(si->percpu_cluster->next[order]);
> > if (offset) {
> > - offset = alloc_swap_scan_cluster(si, offset, &found, order, usage);
> > + ci = lock_cluster(si, offset);
> > + /* Cluster could have been used by another order */
> > + if (cluster_is_usable(ci, order)) {
> > + if (cluster_is_free(ci))
> > + offset = cluster_offset(si, ci);
> > + offset = alloc_swap_scan_cluster(si, offset, &found,
> > + order, usage);
> > + } else {
> > + unlock_cluster(ci);
> > + }
> > if (found)
> > goto done;
> > }
> >
> > - if (!list_empty(&si->free_clusters)) {
> > - ci = list_first_entry(&si->free_clusters, struct swap_cluster_info, list);
> > - offset = alloc_swap_scan_cluster(si, cluster_offset(si, ci), &found, order, usage);
> > - /*
> > - * Either we didn't touch the cluster due to swapoff,
> > - * or the allocation must success.
> > - */
> > - VM_BUG_ON((si->flags & SWP_WRITEOK) && !found);
> > - goto done;
> > +new_cluster:
> > + ci = cluster_isolate_lock(si, &si->free_clusters);
> > + if (ci) {
> > + offset = alloc_swap_scan_cluster(si, cluster_offset(si, ci),
> > + &found, order, usage);
> > + if (found)
> > + goto done;
> > }
> >
> > /* Try reclaim from full clusters if free clusters list is drained */
> > @@ -821,49 +908,45 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
> > swap_reclaim_full_clusters(si, false);
> >
> > if (order < PMD_ORDER) {
> > - unsigned int frags = 0;
> > + unsigned int frags = 0, frags_existing;
> >
> > - while (!list_empty(&si->nonfull_clusters[order])) {
> > - ci = list_first_entry(&si->nonfull_clusters[order],
> > - struct swap_cluster_info, list);
> > - cluster_move(si, ci, &si->frag_clusters[order], CLUSTER_FLAG_FRAG);
> > + while ((ci = cluster_isolate_lock(si, &si->nonfull_clusters[order]))) {
> > offset = alloc_swap_scan_cluster(si, cluster_offset(si, ci),
> > &found, order, usage);
> > - frags++;
> > + /*
> > + * With `fragmenting` set to true, it will surely take
> ~~~~~~~~~~~
> wondering what 'fragmenting' means here.

This comment is a bit out of context indeed, it actually trying to say
the alloc_swap_scan_cluster call above should move the cluster to
tail. I'll update the comment.

>
> > + * the cluster off nonfull list
> > + */
> > if (found)
> > goto done;
> > + frags++;
> > }
> >
> > - /*
> > - * Nonfull clusters are moved to frag tail if we reached
> > - * here, count them too, don't over scan the frag list.
> > - */
> > - while (frags < si->frag_cluster_nr[order]) {
> > - ci = list_first_entry(&si->frag_clusters[order],
> > - struct swap_cluster_info, list);
> > + frags_existing = atomic_long_read(&si->frag_cluster_nr[order]);
> > + while (frags < frags_existing &&
> > + (ci = cluster_isolate_lock(si, &si->frag_clusters[order]))) {
> > + atomic_long_dec(&si->frag_cluster_nr[order]);
> > /*
> > - * Rotate the frag list to iterate, they were all failing
> > - * high order allocation or moved here due to per-CPU usage,
> > - * this help keeping usable cluster ahead.
> > + * Rotate the frag list to iterate, they were all
> > + * failing high order allocation or moved here due to
> > + * per-CPU usage, but they could contain newly released
> > + * reclaimable (eg. lazy-freed swap cache) slots.
> > */
> > - list_move_tail(&ci->list, &si->frag_clusters[order]);
> > offset = alloc_swap_scan_cluster(si, cluster_offset(si, ci),
> > &found, order, usage);
> > - frags++;
> > if (found)
> > goto done;
> > + frags++;
> > }
> > }
> >
> > - 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
> > - * reread cluster_next_cpu since we dropped si->lock
> > - */
> > - swap_do_scheduled_discard(si);
> > + /*
> > + * We don't have free cluster but have some clusters in
> > + * discarding, do discard now and reclaim them, then
> > + * reread cluster_next_cpu since we dropped si->lock
> > + */
> > + if ((si->flags & SWP_PAGE_DISCARD) && swap_do_scheduled_discard(si))
> > goto new_cluster;
> > - }
> >
> > if (order)
> > goto done;
> .....
>
>