[RFC PATCH v1] mm: zswap: Store large folios without splitting

[Ryan Roberts]

Previously zswap would refuse to store any folio bigger than order-0,
and therefore all of those folios would be sent directly to the swap
file. This is a minor inconvenience since swap can currently only
support order-0 and PMD-sized THP, but with the pending introduction of
"small-sized THP", and corresponding changes to swapfile to support any
order of folio, these large folios will become more prevalent and
without this zswap change, zswap will become unusable. Independently of
the "small-sized THP" feature, this change makes it possible to store
existing PMD-sized THPs in zswap.

Modify zswap_store() to allow storing large folios. The function is
split into 2 parts; zswap_store() does all the per-folio operations
(i.e. checking there is enough space, etc). Then it calls a new helper,
zswap_store_page(), for each page in the folio, which are stored as
their own entries in the zswap pool. (These entries continue to be
loaded back individually as single pages). If a store fails for any
single page, then all previously successfully stored folio pages are
invalidated.

Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
I've tested this on arm64 (m2) with zswap enabled, and running
vm-scalability's `usemem` across multiple cores from within a
memory-constrained memcg to force high volumes of swap. I've also run mm
selftests and observe no regressions (although there is nothing [z]swap
specific there) - does zswap have any specific tests I should run?

This is based on mm-stable, since mm-unstable contains a zswap patch
known to be buggy [1]. I thought it would be best to get comments on the
shape, then do the rebase after that patch has been fixed.

For context, small-sized THP is being discussed here [2], and I'm
working on changes to swapfile to support non-PMD-sized large folios
here [3].

[1] https://lore.kernel.org/linux-mm/21606fe5-fb9b-4d37-98ab-38c96819893b@xxxxxxx/
[2] https://lore.kernel.org/linux-mm/20230929114421.3761121-1-ryan.roberts@xxxxxxx/
[3] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@xxxxxxx/

Thanks,
Ryan


 mm/zswap.c | 155 +++++++++++++++++++++++++++++++++--------------------
 1 file changed, 98 insertions(+), 57 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 37d2b1cb2ecb..51cbfc4e1ef8 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1188,18 +1188,17 @@ static void zswap_fill_page(void *ptr, unsigned long value)
 	memset_l(page, value, PAGE_SIZE / sizeof(unsigned long));
 }

-bool zswap_store(struct folio *folio)
+static bool zswap_store_page(struct folio *folio, long index,
+			     struct obj_cgroup *objcg, struct zswap_pool *pool)
 {
 	swp_entry_t swp = folio->swap;
 	int type = swp_type(swp);
-	pgoff_t offset = swp_offset(swp);
-	struct page *page = &folio->page;
+	pgoff_t offset = swp_offset(swp) + index;
+	struct page *page = folio_page(folio, index);
 	struct zswap_tree *tree = zswap_trees[type];
 	struct zswap_entry *entry, *dupentry;
 	struct scatterlist input, output;
 	struct crypto_acomp_ctx *acomp_ctx;
-	struct obj_cgroup *objcg = NULL;
-	struct zswap_pool *pool;
 	struct zpool *zpool;
 	unsigned int dlen = PAGE_SIZE;
 	unsigned long handle, value;
@@ -1208,51 +1207,11 @@ bool zswap_store(struct folio *folio)
 	gfp_t gfp;
 	int ret;

-	VM_WARN_ON_ONCE(!folio_test_locked(folio));
-	VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
-
-	/* Large folios aren't supported */
-	if (folio_test_large(folio))
-		return false;
-
-	if (!zswap_enabled || !tree)
+	/* entry keeps the references if successfully stored. */
+	if (!zswap_pool_get(pool))
 		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
-	 * the tree, and it might be written back overriding the new data.
-	 */
-	spin_lock(&tree->lock);
-	dupentry = zswap_rb_search(&tree->rbroot, offset);
-	if (dupentry) {
-		zswap_duplicate_entry++;
-		zswap_invalidate_entry(tree, dupentry);
-	}
-	spin_unlock(&tree->lock);
-
-	/*
-	 * XXX: zswap reclaim does not work with cgroups yet. Without a
-	 * cgroup-aware entry LRU, we will push out entries system-wide based on
-	 * local cgroup limits.
-	 */
-	objcg = get_obj_cgroup_from_folio(folio);
-	if (objcg && !obj_cgroup_may_zswap(objcg))
-		goto reject;
-
-	/* reclaim space if needed */
-	if (zswap_is_full()) {
-		zswap_pool_limit_hit++;
-		zswap_pool_reached_full = true;
-		goto shrink;
-	}
-
-	if (zswap_pool_reached_full) {
-	       if (!zswap_can_accept())
-			goto shrink;
-		else
-			zswap_pool_reached_full = false;
-	}
+	if (objcg)
+		obj_cgroup_get(objcg);

 	/* allocate entry */
 	entry = zswap_entry_cache_alloc(GFP_KERNEL);
@@ -1260,6 +1219,8 @@ bool zswap_store(struct folio *folio)
 		zswap_reject_kmemcache_fail++;
 		goto reject;
 	}
+	entry->objcg = objcg;
+	entry->pool = pool;

 	if (zswap_same_filled_pages_enabled) {
 		src = kmap_atomic(page);
@@ -1277,11 +1238,6 @@ bool zswap_store(struct folio *folio)
 	if (!zswap_non_same_filled_pages_enabled)
 		goto freepage;

-	/* if entry is successfully added, it keeps the reference */
-	entry->pool = zswap_pool_current_get();
-	if (!entry->pool)
-		goto freepage;
-
 	/* compress */
 	acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);

@@ -1337,7 +1293,6 @@ bool zswap_store(struct folio *folio)
 	entry->length = dlen;

 insert_entry:
-	entry->objcg = objcg;
 	if (objcg) {
 		obj_cgroup_charge_zswap(objcg, entry->length);
 		/* Account before objcg ref is moved to tree */
@@ -1373,19 +1328,105 @@ bool zswap_store(struct folio *folio)

 put_dstmem:
 	mutex_unlock(acomp_ctx->mutex);
-	zswap_pool_put(entry->pool);
 freepage:
 	zswap_entry_cache_free(entry);
 reject:
 	if (objcg)
 		obj_cgroup_put(objcg);
+	zswap_pool_put(pool);
 	return false;
+}

+bool zswap_store(struct folio *folio)
+{
+	long nr_pages = folio_nr_pages(folio);
+	swp_entry_t swp = folio->swap;
+	int type = swp_type(swp);
+	pgoff_t offset = swp_offset(swp);
+	struct zswap_tree *tree = zswap_trees[type];
+	struct zswap_entry *entry;
+	struct obj_cgroup *objcg = NULL;
+	struct zswap_pool *pool;
+	bool ret = false;
+	long i;
+
+	VM_WARN_ON_ONCE(!folio_test_locked(folio));
+	VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
+
+	if (!zswap_enabled || !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
+	 * the tree, and it might be written back overriding the new data.
+	 */
+	spin_lock(&tree->lock);
+	for (i = 0; i < nr_pages; i++) {
+		entry = zswap_rb_search(&tree->rbroot, offset + i);
+		if (entry) {
+			zswap_duplicate_entry++;
+			zswap_invalidate_entry(tree, entry);
+		}
+	}
+	spin_unlock(&tree->lock);
+
+	/*
+	 * XXX: zswap reclaim does not work with cgroups yet. Without a
+	 * cgroup-aware entry LRU, we will push out entries system-wide based on
+	 * local cgroup limits.
+	 */
+	objcg = get_obj_cgroup_from_folio(folio);
+	if (objcg && !obj_cgroup_may_zswap(objcg))
+		goto put_objcg;
+
+	/* reclaim space if needed */
+	if (zswap_is_full()) {
+		zswap_pool_limit_hit++;
+		zswap_pool_reached_full = true;
+		goto shrink;
+	}
+
+	if (zswap_pool_reached_full) {
+		if (!zswap_can_accept())
+			goto shrink;
+		else
+			zswap_pool_reached_full = false;
+	}
+
+	pool = zswap_pool_current_get();
+	if (!pool)
+		goto put_objcg;
+
+	/*
+	 * Store each page of the folio as a separate entry. If we fail to store
+	 * a page, unwind by removing all the previous pages we stored.
+	 */
+	for (i = 0; i < nr_pages; i++) {
+		if (!zswap_store_page(folio, i, objcg, pool)) {
+			spin_lock(&tree->lock);
+			for (i--; i >= 0; i--) {
+				entry = zswap_rb_search(&tree->rbroot, offset + i);
+				if (entry)
+					zswap_invalidate_entry(tree, entry);
+			}
+			spin_unlock(&tree->lock);
+			goto put_pool;
+		}
+	}
+
+	ret = true;
+put_pool:
+	zswap_pool_put(pool);
+put_objcg:
+	if (objcg)
+		obj_cgroup_put(objcg);
+	return ret;
 shrink:
 	pool = zswap_pool_last_get();
 	if (pool && !queue_work(shrink_wq, &pool->shrink_work))
 		zswap_pool_put(pool);
-	goto reject;
+	goto put_objcg;
 }

 bool zswap_load(struct folio *folio)

base-commit: 158978945f3173b8c1a88f8c5684a629736a57ac
--
2.25.1