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

Kairui Song
2025-01-13T05:49:44Z

On Mon, Jan 6, 2025 at 4:43 PM Baoquan He <bhe@redhat.com> wrote:
>
> 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.

Hi Baoquan,

Not sure what you mean. The flags are exclusive, and after this
commit, we are always using "ci->flags == CLUSTER_FLAG_XXX" to check
the flag.

>
> >
> > 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.

Good idea.

>
> 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
> >
> >
>
>