Re: [PATCH 1/2] mm: zswap.c: add xarray tree to zswap

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Jan 17, 2024 at 7:06 PM Chris Li <chrisl@kernel.org> wrote:
>
> The xarray tree is added alongside the zswap RB tree.
> Checks for the xarray get the same result as the RB tree operations.
>
> Rename the zswap RB tree function to a more generic function
> name without the RB part.

As I mentioned in the cover letter, I believe this should be squashed
into the second patch. I have some comments below as well on the parts
that should remain after the squash.

[..]
>
> @@ -462,9 +463,9 @@ static void zswap_lru_putback(struct list_lru *list_lru,
>  /*********************************
>  * rbtree functions
>  **********************************/
> -static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
> +static struct zswap_entry *zswap_search(struct zswap_tree *tree, pgoff_t offset)

Let's change the zswap_rb_* prefixes to zswap_tree_* instead of just
zswap_*. Otherwise, it will be confusing to have both zswap_store and
zswap_insert (as well as zswap_load and zswap_search).

[..]
> @@ -1790,15 +1808,21 @@ void zswap_swapon(int type)
>  void zswap_swapoff(int type)
>  {
>         struct zswap_tree *tree = zswap_trees[type];
> -       struct zswap_entry *entry, *n;
> +       struct zswap_entry *entry, *e, *n;
> +       XA_STATE(xas, tree ? &tree->xarray : NULL, 0);
>
>         if (!tree)
>                 return;
>
>         /* walk the tree and free everything */
>         spin_lock(&tree->lock);
> +
> +       xas_for_each(&xas, e, ULONG_MAX)

Why not use xa_for_each?

> +               zswap_invalidate_entry(tree, e);
> +
>         rbtree_postorder_for_each_entry_safe(entry, n, &tree->rbroot, rbnode)
> -               zswap_free_entry(entry);

Replacing zswap_free_entry() with zswap_invalidate_entry() is a
behavioral change that should be done separate from this series, but I
am wondering why it's needed. IIUC, the swapoff code should be making
sure there are no ongoing swapin/swapout operations, and there are no
pages left in zswap to writeback.

Is it the case that swapoff may race with writeback, such that
writeback is holding the last remaining ref after zswap_invalidate()
is called, and then zswap_swapoff() is called freeing the zswap entry
while writeback is still accessing it?