Re: [PATCH v2] zswap: replace RB tree with xarray

From: Chris Li
Date: Thu Feb 29 2024 - 13:58:57 EST

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In reply to: Chengming Zhou: "Re: [PATCH v2] zswap: replace RB tree with xarray"
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Hi Chengming,

Thanks for the review and feedback.

On Thu, Feb 29, 2024 at 1:44 AM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> Hi Chris,
>
> On 2024/2/29 16:46, Chris Li wrote:
> > Very deep RB tree requires rebalance at times. That
> > contributes to the zswap fault latencies. Xarray does not
> > need to perform tree rebalance. Replacing RB tree to xarray
> > can have some small performance gain.
> >
> > One small difference is that xarray insert might fail with
> > ENOMEM, while RB tree insert does not allocate additional
> > memory.
> >
> > The zswap_entry size will reduce a bit due to removing the
> > RB node, which has two pointers and a color field. Xarray
> > store the pointer in the xarray tree rather than the
> > zswap_entry. Every entry has one pointer from the xarray
> > tree. Overall, switching to xarray should save some memory,
> > if the swap entries are densely packed.
> >
> > Notice the zswap_rb_search and zswap_rb_insert always
> > followed by zswap_rb_erase. Fold the entry erase into
> > zswap_xa_search_and_erase and zswap_xa_insert. That saves
> > one tree lookup as well.
> >
> > Remove zswap_invalidate_entry due to no need to call
> > zswap_rb_erase any more. Use zswap_free_entry instead.
> >
> > The "struct zswap_tree" has been replaced by "struct xarray".
> > The tree spin lock has transferred to the xarray lock.
> >
> > Thanks to Chengming for providing the kernel build test number.
> >
> > Run the kernel build testing 5 times for each version, averages:
> > (memory.max=2GB, zswap shrinker and writeback enabled, one 50GB swapfile.)
> >
> > mm-266f922c0b5e zswap-xarray-test
> > real 63.43 63.12
> > user 1063.78 1062.59
> > sys 272.49 265.66
> >
> > The sys time is about 2.5% faster.
> >
> > Tested-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> > ---
> >
> >
> > Signed-off-by: Chris Li <chrisl@xxxxxxxxxx>
> > ---
> > Changes in v2:
> > - Replace struct zswap_tree with struct xarray.
> > - Remove zswap_tree spinlock, use xarray lock instead.
> > - Fold zswap_rb_erase() into zswap_xa_search_and_delete() and zswap_xa_insert().
> > - Delete zswap_invalidate_entry(), use zswap_free_entry() instead.
> > - Link to v1: https://lore.kernel.org/r/20240117-zswap-xarray-v1-0-6daa86c08fae@xxxxxxxxxx
> > ---
> > mm/zswap.c | 173 +++++++++++++++++++++++--------------------------------------
> > 1 file changed, 64 insertions(+), 109 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 011e068eb355..ac9ef14d88be 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -20,7 +20,6 @@
> > #include <linux/spinlock.h>
> > #include <linux/types.h>
> > #include <linux/atomic.h>
> > -#include <linux/rbtree.h>
> > #include <linux/swap.h>
> > #include <linux/crypto.h>
> > #include <linux/scatterlist.h>
> > @@ -71,6 +70,8 @@ static u64 zswap_reject_compress_poor;
> > static u64 zswap_reject_alloc_fail;
> > /* Store failed because the entry metadata could not be allocated (rare) */
> > static u64 zswap_reject_kmemcache_fail;
> > +/* Store failed because xarray can't insert the entry*/
> > +static u64 zswap_reject_xarray_fail;
> >
> > /* Shrinker work queue */
> > static struct workqueue_struct *shrink_wq;
> > @@ -196,7 +197,6 @@ static struct {
> > * This structure contains the metadata for tracking a single compressed
> > * page within zswap.
> > *
> > - * rbnode - links the entry into red-black tree for the appropriate swap type
> > * swpentry - associated swap entry, the offset indexes into the red-black tree
> > * length - the length in bytes of the compressed page data. Needed during
> > * decompression. For a same value filled page length is 0, and both
> > @@ -208,7 +208,6 @@ static struct {
> > * lru - handle to the pool's lru used to evict pages.
> > */
> > struct zswap_entry {
> > - struct rb_node rbnode;
> > swp_entry_t swpentry;
> > unsigned int length;
> > struct zswap_pool *pool;
> > @@ -220,12 +219,7 @@ struct zswap_entry {
> > struct list_head lru;
> > };
> >
> > -struct zswap_tree {
> > - struct rb_root rbroot;
> > - spinlock_t lock;
> > -};
> > -
> > -static struct zswap_tree *zswap_trees[MAX_SWAPFILES];
> > +static struct xarray *zswap_trees[MAX_SWAPFILES];
> > static unsigned int nr_zswap_trees[MAX_SWAPFILES];
> >
> > /* RCU-protected iteration */
> > @@ -253,10 +247,10 @@ static bool zswap_has_pool;
> > * helpers and fwd declarations
> > **********************************/
> >
> > -static inline struct zswap_tree *swap_zswap_tree(swp_entry_t swp)
> > +static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
> > {
> > - return &zswap_trees[swp_type(swp)][swp_offset(swp)
> > - >> SWAP_ADDRESS_SPACE_SHIFT];
> > + return zswap_trees[swp_type(swp)] + (swp_offset(swp)
> > + >> SWAP_ADDRESS_SPACE_SHIFT);
> > }
> >
> > #define zswap_pool_debug(msg, p) \
> > @@ -805,60 +799,38 @@ void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
> > }
> >
> > /*********************************
> > -* rbtree functions
> > +* xarray functions
> > **********************************/
> > -static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
> > +static struct zswap_entry *zswap_xa_search_and_erase(struct xarray *tree, pgoff_t offset)
> > {
> > - struct rb_node *node = root->rb_node;
> > - struct zswap_entry *entry;
> > - pgoff_t entry_offset;
> > -
> > - while (node) {
> > - entry = rb_entry(node, struct zswap_entry, rbnode);
> > - entry_offset = swp_offset(entry->swpentry);
> > - if (entry_offset > offset)
> > - node = node->rb_left;
> > - else if (entry_offset < offset)
> > - node = node->rb_right;
> > - else
> > - return entry;
> > - }
> > - return NULL;
> > + return xa_erase(tree, offset);
> > }
> >
> > /*
> > + * Expects xa_lock to be held on entry.
> > * In the case that a entry with the same offset is found, a pointer to
> > - * the existing entry is stored in dupentry and the function returns -EEXIST
> > + * the existing entry is stored in old and erased from the tree.
> > + * Function return error on insert.
> > */
> > -static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
> > - struct zswap_entry **dupentry)
> > +static int zswap_xa_insert(struct xarray *tree, struct zswap_entry *entry,
> > + struct zswap_entry **old)
> > {
> > - struct rb_node **link = &root->rb_node, *parent = NULL;
> > - struct zswap_entry *myentry;
> > - 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 {
> > - *dupentry = myentry;
> > - return -EEXIST;
> > - }
> > - }
> > - rb_link_node(&entry->rbnode, parent, link);
> > - rb_insert_color(&entry->rbnode, root);
> > - return 0;
> > -}
> > + int err;
> > + struct zswap_entry *e;
> > + pgoff_t offset = swp_offset(entry->swpentry);
> >
> > -static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
> > -{
> > - rb_erase(&entry->rbnode, root);
> > - RB_CLEAR_NODE(&entry->rbnode);
> > + e = __xa_store(tree, offset, entry, GFP_KERNEL);
> > + err = xa_err(e);
> > +
> > + if (err) {
> > + e = __xa_erase(tree, offset);

zswap_xa_insert will always erase the old entry, even when __xa_store fails.

> > + if (err == -ENOMEM)
> > + zswap_reject_alloc_fail++;
> > + else
> > + zswap_reject_xarray_fail++;
> > + }
> > + *old = e;

Old pointer is set regardless of the error.

> > + return err;
> > }
> >
> > /*********************************
> > @@ -872,7 +844,6 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp, int nid)
> > entry = kmem_cache_alloc_node(zswap_entry_cache, gfp, nid);
> > if (!entry)
> > return NULL;
> > - RB_CLEAR_NODE(&entry->rbnode);
> > return entry;
> > }
> >
> > @@ -914,17 +885,6 @@ static void zswap_entry_free(struct zswap_entry *entry)
> > zswap_update_total_size();
> > }
> >
> > -/*
> > - * The caller hold the tree lock and search the entry from the tree,
> > - * so it must be on the tree, remove it from the tree and free it.
> > - */
> > -static void zswap_invalidate_entry(struct zswap_tree *tree,
> > - struct zswap_entry *entry)
> > -{
> > - zswap_rb_erase(&tree->rbroot, entry);
> > - zswap_entry_free(entry);
> > -}
> > -
> > /*********************************
> > * compressed storage functions
> > **********************************/
> > @@ -1113,7 +1073,9 @@ static void zswap_decompress(struct zswap_entry *entry, struct page *page)
> > static int zswap_writeback_entry(struct zswap_entry *entry,
> > swp_entry_t swpentry)
> > {
> > - struct zswap_tree *tree;
> > + struct xarray *tree;
> > + pgoff_t offset = swp_offset(swpentry);
> > + struct zswap_entry *e;
> > struct folio *folio;
> > struct mempolicy *mpol;
> > bool folio_was_allocated;
> > @@ -1150,19 +1112,14 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
> > * be dereferenced.
> > */
> > tree = swap_zswap_tree(swpentry);
> > - spin_lock(&tree->lock);
> > - if (zswap_rb_search(&tree->rbroot, swp_offset(swpentry)) != entry) {
> > - spin_unlock(&tree->lock);
> > + e = zswap_xa_search_and_erase(tree, offset);
> > + if (e != entry) {
>
> IIUC, here we should use xa_cmpxchg() instead of erasing it unconditionally.

Good catch, I agree with your suggestion. I will spin a V3 to correct that.

>
> > delete_from_swap_cache(folio);
> > folio_unlock(folio);
> > folio_put(folio);
> > return -ENOMEM;
> > }
> >
> > - /* Safe to deref entry after the entry is verified above. */
> > - zswap_rb_erase(&tree->rbroot, entry);
> > - spin_unlock(&tree->lock);
> > -
> > zswap_decompress(entry, &folio->page);
> >
> > count_vm_event(ZSWPWB);
> > @@ -1471,10 +1428,11 @@ bool zswap_store(struct folio *folio)
> > {
> > swp_entry_t swp = folio->swap;
> > pgoff_t offset = swp_offset(swp);
> > - struct zswap_tree *tree = swap_zswap_tree(swp);
> > - struct zswap_entry *entry, *dupentry;
> > + struct xarray *tree = swap_zswap_tree(swp);
> > + struct zswap_entry *entry, *old;
> > struct obj_cgroup *objcg = NULL;
> > struct mem_cgroup *memcg = NULL;
> > + int err;
> >
> > VM_WARN_ON_ONCE(!folio_test_locked(folio));
> > VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> > @@ -1562,21 +1520,25 @@ bool zswap_store(struct folio *folio)
> > }
> >
> > /* map */
> > - spin_lock(&tree->lock);
> > + xa_lock(tree);
> > /*
> > * The folio may have been dirtied again, invalidate the
> > * possibly stale entry before inserting the new entry.
> > */
> > - if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
> > - zswap_invalidate_entry(tree, dupentry);
> > - WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry));
> > + err = zswap_xa_insert(tree, entry, &old);
> > + if (old)
> > + zswap_entry_free(old);
>
> Maybe it's safer to check old after !err, since "old" variable is not initialized
> to NULL, and zswap_xa_insert() maybe won't overwrite "old" to NULL when err return?

That is the intended behavior.

See the above in zswap_xa_insert(). It will always erase and return
"old" even when the __xa_store() has an error.
That is because by the time zswap needs to store a new entry at this
swap entry. The old data is already outdated. We should just remove
the old data. If __xa_store failed due to out of memory. That is the
same as allocating an entry out of memory. It is fine to fail
swap_store. Then the folio will just stay in the swap cache for the
next time.

Do you see any ill effects can be caused by deleting the old entry on
xa_insert error?

> > + if (err) {
> > + xa_unlock(tree);
> > + goto free_zpool;
> > }
> > +
> > if (entry->length) {
> > INIT_LIST_HEAD(&entry->lru);
> > zswap_lru_add(&zswap.list_lru, entry);
> > atomic_inc(&zswap.nr_stored);
> > }
>
> It seems that we can put this part out of the xarray lock section, then it's enough to
> just use xa_insert().

It is not enough protection. Consider this race:

CPU1 CPU2

xa_insert()
entry = swap_xa_search_and_erase()
zswap_free_entry(entry)

if (entry->length)
...
CPU1 is using entry after free.

Chris

Next message: Fan Wu: "Re: [RFC PATCH v13 17/20] ipe: enable support for fs-verity as a trust provider"
Previous message: William Breathitt Gray: "Re: [PATCH v3 04/10] counter: stm32-timer-cnt: introduce clock signal"
In reply to: Chengming Zhou: "Re: [PATCH v2] zswap: replace RB tree with xarray"
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