mm/zswap: make sure each swapfile always have zswap rb-tree

Patch series "mm/zswap: optimize the scalability of zswap rb-tree", v2.

When testing the zswap performance by using kernel build -j32 in a tmpfs
directory, I found the scalability of zswap rb-tree is not good, which is
protected by the only spinlock.  That would cause heavy lock contention if
multiple tasks zswap_store/load concurrently.

So a simple solution is to split the only one zswap rb-tree into multiple
rb-trees, each corresponds to SWAP_ADDRESS_SPACE_PAGES (64M).  This idea
is from the commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB
trunks").

Although this method can't solve the spinlock contention completely, it
can mitigate much of that contention.  Below is the results of kernel
build in tmpfs with zswap shrinker enabled:

     linux-next  zswap-lock-optimize
real 1m9.181s    1m3.820s
user 17m44.036s  17m40.100s
sys  7m37.297s   4m54.622s

So there are clearly improvements.  And it's complementary with the
ongoing zswap xarray conversion by Chris.  Anyway, I think we can also
merge this first, it's complementary IMHO.  So I just refresh and resend
this for further discussion.


This patch (of 2):

Not all zswap interfaces can handle the absence of the zswap rb-tree,
actually only zswap_store() has handled it for now.

To make things simple, we make sure each swapfile always have the zswap
rb-tree prepared before being enabled and used.  The preparation is
unlikely to fail in practice, this patch just make it explicit.

Link: https://lkml.kernel.org/r/20240117-b4-zswap-lock-optimize-v2-0-b5cc55479090@bytedance.com
Link: https://lkml.kernel.org/r/20240117-b4-zswap-lock-optimize-v2-1-b5cc55479090@bytedance.com
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Chris Li <chriscli@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 3 insertions, 5 deletions

diff --git a/mm/zswap.c b/mm/zswap.c
index 2c3d77c6fe72..5a40a7b4bae8 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1518,9 +1518,6 @@ bool zswap_store(struct folio *folio)
 	if (folio_test_large(folio))
 		return false;
 
-	if (!tree)
-		return false;
-
 	/*
 	 * If this is a duplicate, it must be removed before attempting to store
 	 * it, otherwise, if the store fails the old page won't be removed from
@@ -1775,19 +1772,20 @@ void zswap_invalidate(int type, pgoff_t offset)
 	spin_unlock(&tree->lock);
 }
 
-void zswap_swapon(int type)
+int zswap_swapon(int type)
 {
 	struct zswap_tree *tree;
 
 	tree = kzalloc(sizeof(*tree), GFP_KERNEL);
 	if (!tree) {
 		pr_err("alloc failed, zswap disabled for swap type %d\n", type);
-		return;
+		return -ENOMEM;
 	}
 
 	tree->rbroot = RB_ROOT;
 	spin_lock_init(&tree->lock);
 	zswap_trees[type] = tree;
+	return 0;
 }
 
 void zswap_swapoff(int type)