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Howlett" , Joel Fernandes , Chengming Zhou Subject: Re: [PATCH 1/2] mm: zswap.c: add xarray tree to zswap Message-ID: References: <20240117-zswap-xarray-v1-0-6daa86c08fae@kernel.org> <20240117-zswap-xarray-v1-1-6daa86c08fae@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: > > > -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). > How about zswap_xa_* ? SGTM. > > > > [..] > > > @@ -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? > For the RB tree the mapping is stored in the zswap entry as RB node. > That is different from xarray. Xarry stores the mapping outside of > zswap entry. Just freeing the entry does not remove the mapping from > xarray. Therefore it needs to call zswap_invalidate_entry() to remove > the entry from the xarray. I could call zswap_erase() then free entry. > I just think zswap_invalidate_entry() is more consistent with the rest > of the code. Do we have to call xa_destroy() anyway to make sure everything is cleaned up in the xarray? In that case, we can just do that after the loop.