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    SubjectRe: [PATCH 2/2] mm: zswap.c: remove RB tree
    On Wed, Jan 17, 2024 at 10:35 PM Yosry Ahmed <yosryahmed@google.com> wrote:
    >
    > > @@ -493,45 +471,47 @@ static struct zswap_entry *zswap_search(struct zswap_tree *tree, pgoff_t offset)
    > > static int zswap_insert(struct zswap_tree *tree, struct zswap_entry *entry,
    > > struct zswap_entry **dupentry)
    > > {
    > > - struct rb_root *root = &tree->rbroot;
    > > - struct rb_node **link = &root->rb_node, *parent = NULL;
    > > - struct zswap_entry *myentry, *old;
    > > - pgoff_t myentry_offset, entry_offset = swp_offset(entry->swpentry);
    > > -
    > > -
    > > - while (*link) {
    > > - parent = *link;
    > > - myentry = rb_entry(parent, struct zswap_entry, rbnode);
    > > - myentry_offset = swp_offset(myentry->swpentry);
    > > - if (myentry_offset > entry_offset)
    > > - link = &(*link)->rb_left;
    > > - else if (myentry_offset < entry_offset)
    > > - link = &(*link)->rb_right;
    > > - else {
    > > - old = xa_load(&tree->xarray, entry_offset);
    > > - BUG_ON(old != myentry);
    > > - *dupentry = myentry;
    > > + struct zswap_entry *e;
    > > + pgoff_t offset = swp_offset(entry->swpentry);
    > > + XA_STATE(xas, &tree->xarray, offset);
    > > +
    > > + do {
    > > + xas_lock_irq(&xas);
    > > + do {
    > > + e = xas_load(&xas);
    > > + if (xa_is_zero(e))
    > > + e = NULL;
    > > + } while (xas_retry(&xas, e));
    > > + if (xas_valid(&xas) && e) {
    > > + xas_unlock_irq(&xas);
    > > + *dupentry = e;
    > > return -EEXIST;
    > > }
    > > - }
    > > - rb_link_node(&entry->rbnode, parent, link);
    > > - rb_insert_color(&entry->rbnode, root);
    > > - old = xa_store(&tree->xarray, entry_offset, entry, GFP_KERNEL);
    > > - return 0;
    > > + xas_store(&xas, entry);
    > > + xas_unlock_irq(&xas);
    > > + } while (xas_nomem(&xas, GFP_KERNEL));
    > > + return xas_error(&xas);
    >
    > I think using the xas_* APIs can be avoided here. The only reason we
    > need it is that we want to check if there's an existing entry first,
    > and return -EEXIST. However, in that case, the caller will replace it
    > anyway (and do some operations on the dupentry):
    >
    > while (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
    > WARN_ON(1);
    > zswap_duplicate_entry++;
    > zswap_invalidate_entry(tree, dupentry);
    > }
    >
    > So I think we can do something like this in zswap_insert() instead:
    >
    > dupentry = xa_store(..);
    > if (WARN_ON(dupentry)) {
    > zswap_duplicate_entry++;
    > zswap_invalidate_entry(tree, dupentry);
    > }

    If this is doable, I think we can return xa_store(..) and keep the
    logic in the caller. I think there's a chance
    zswap_{search/insert/erase} may end up being very thin wrappers around
    xa_{load/store/erase}, and we may be able to remove them completely.
    Let's see how it goes.

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