mm: kill frontswap

The only user of frontswap is zswap, and has been for a long time.  Have
swap call into zswap directly and remove the indirection.

[hannes@cmpxchg.org: remove obsolete comment, per Yosry]
  Link: https://lkml.kernel.org/r/20230719142832.GA932528@cmpxchg.org
[fengwei.yin@intel.com: don't warn if none swapcache folio is passed to zswap_load]
  Link: https://lkml.kernel.org/r/20230810095652.3905184-1-fengwei.yin@intel.com
Link: https://lkml.kernel.org/r/20230717160227.GA867137@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 37 insertions, 0 deletions

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
new file mode 100644
index 000000000000..850c377d9b6d
--- /dev/null
+++ b/include/linux/zswap.h
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_ZSWAP_H
+#define _LINUX_ZSWAP_H
+
+#include <linux/types.h>
+#include <linux/mm_types.h>
+
+extern u64 zswap_pool_total_size;
+extern atomic_t zswap_stored_pages;
+
+#ifdef CONFIG_ZSWAP
+
+bool zswap_store(struct page *page);
+bool zswap_load(struct page *page);
+void zswap_invalidate(int type, pgoff_t offset);
+void zswap_swapon(int type);
+void zswap_swapoff(int type);
+
+#else
+
+static inline bool zswap_store(struct page *page)
+{
+	return false;
+}
+
+static inline bool zswap_load(struct page *page)
+{
+	return false;
+}
+
+static inline void zswap_invalidate(int type, pgoff_t offset) {}
+static inline void zswap_swapon(int type) {}
+static inline void zswap_swapoff(int type) {}
+
+#endif
+
+#endif /* _LINUX_ZSWAP_H */