[PATCH v8 3/3] arm64: mte: add compression support to mteswap.c

[Alexander Potapenko]

Update mteswap.c to perform inline compression of memory tags when
possible.

If CONFIG_ARM64_MTE_COMP is enabled, mteswap.c will attemt to compress
saved tags for a struct page and store them directly in Xarray entry
instead of wasting heap space.

Soon after booting Android, tag compression saves ~2x memory previously
spent by mteswap.c on tag allocations. On a moderately loaded device with
~20% tagged pages, this leads to saving several megabytes of kernel heap:

  # cat /sys/kernel/debug/mteswap/stats
  8 bytes: 102496 allocations, 67302 deallocations
  128 bytes: 212234 allocations, 178278 deallocations
  uncompressed tag storage size: 8851200
  compressed tag storage size: 4346368

Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>

---
 v8:
  - adapt to the new compression API, abandon mteswap_{no,}comp.c
  - move stats collection to mteswap.c

 v5:
  - drop a dead variable from _mte_free_saved_tags() in mteswap_comp.c
  - ensure MTE compression works with arbitrary page sizes
  - update patch description

 v4:
  - minor code simplifications suggested by Andy Shevchenko, added
    missing header dependencies
  - changed compression API names to reflect modifications made to
    memcomp.h (as suggested by Yury Norov)

 v3:
  - Addressed comments by Andy Shevchenko in another patch:
   - fixed includes order
   - replaced u64 with unsigned long
   - added MODULE_IMPORT_NS(MTECOMP)
---
 arch/arm64/mm/mteswap.c | 88 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 84 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index a31833e3ddc54..0f558942d88b8 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -1,28 +1,48 @@
 // SPDX-License-Identifier: GPL-2.0-only
 
+#include <linux/debugfs.h>
 #include <linux/pagemap.h>
 #include <linux/xarray.h>
 #include <linux/slab.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <asm/mte.h>
+#include <asm/mtecomp.h>
+#include "mtecomp.h"
+
+enum mteswap_counters {
+	MTESWAP_CTR_INLINE = 0,
+	MTESWAP_CTR_NOINLINE,
+	MTESWAP_CTR_SIZE
+};
+static atomic_long_t alloc_counters[MTESWAP_CTR_SIZE];
+static atomic_long_t dealloc_counters[MTESWAP_CTR_SIZE];
 
 static DEFINE_XARRAY(mte_pages);
 
 void *mte_allocate_tag_storage(void)
 {
-	/* tags granule is 16 bytes, 2 tags stored per byte */
-	return kmalloc(MTE_PAGE_TAG_STORAGE, GFP_KERNEL);
+	void *ret;
+
+	ret = kmalloc(MTE_PAGE_TAG_STORAGE, GFP_KERNEL);
+	if (ret)
+		atomic_long_inc(&alloc_counters[MTESWAP_CTR_NOINLINE]);
+	return ret;
 }
 
 void mte_free_tag_storage(char *storage)
 {
-	kfree(storage);
+	if (!mte_is_compressed(storage)) {
+		kfree(storage);
+		atomic_long_dec(&alloc_counters[MTESWAP_CTR_NOINLINE]);
+	} else {
+		atomic_long_dec(&alloc_counters[MTESWAP_CTR_INLINE]);
+	}
 }
 
 int mte_save_tags(struct page *page)
 {
-	void *tag_storage, *ret;
+	void *tag_storage, *ret, *compressed;
 
 	if (!page_mte_tagged(page))
 		return 0;
@@ -32,6 +52,12 @@ int mte_save_tags(struct page *page)
 		return -ENOMEM;
 
 	mte_save_page_tags(page_address(page), tag_storage);
+	compressed = mte_compress(tag_storage);
+	if (compressed) {
+		mte_free_tag_storage(tag_storage);
+		tag_storage = (void *)compressed;
+		atomic_long_inc(&alloc_counters[MTESWAP_CTR_INLINE]);
+	}
 
 	/* lookup the swap entry.val from the page */
 	ret = xa_store(&mte_pages, page_swap_entry(page).val, tag_storage,
@@ -50,13 +76,20 @@ int mte_save_tags(struct page *page)
 void mte_restore_tags(swp_entry_t entry, struct page *page)
 {
 	void *tags = xa_load(&mte_pages, entry.val);
+	void *tag_storage = NULL;
 
 	if (!tags)
 		return;
 
 	if (try_page_mte_tagging(page)) {
+		if (mte_is_compressed(tags)) {
+			tag_storage = mte_allocate_tag_storage();
+			mte_decompress(tags, tag_storage);
+			tags = tag_storage;
+		}
 		mte_restore_page_tags(page_address(page), tags);
 		set_page_mte_tagged(page);
+		mte_free_tag_storage(tag_storage);
 	}
 }
 
@@ -83,3 +116,50 @@ void mte_invalidate_tags_area(int type)
 	}
 	xa_unlock(&mte_pages);
 }
+
+/* DebugFS interface. */
+static int stats_show(struct seq_file *seq, void *v)
+{
+	unsigned long total_mem_alloc = 0, total_mem_dealloc = 0;
+	unsigned long total_num_alloc = 0, total_num_dealloc = 0;
+	unsigned long sizes[2] = { 8, MTE_PAGE_TAG_STORAGE };
+	long alloc, dealloc;
+	unsigned long size;
+	int i;
+
+	for (i = 0; i < MTESWAP_CTR_SIZE; i++) {
+		alloc = atomic_long_read(&alloc_counters[i]);
+		dealloc = atomic_long_read(&dealloc_counters[i]);
+		total_num_alloc += alloc;
+		total_num_dealloc += dealloc;
+		size = sizes[i];
+		/*
+		 * Do not count 8-byte buffers towards compressed tag storage
+		 * size.
+		 */
+		if (i) {
+			total_mem_alloc += (size * alloc);
+			total_mem_dealloc += (size * dealloc);
+		}
+		seq_printf(seq,
+			   "%lu bytes: %lu allocations, %lu deallocations\n",
+			   size, alloc, dealloc);
+	}
+	seq_printf(seq, "uncompressed tag storage size: %lu\n",
+		   (total_num_alloc - total_num_dealloc) *
+			   MTE_PAGE_TAG_STORAGE);
+	seq_printf(seq, "compressed tag storage size: %lu\n",
+		   total_mem_alloc - total_mem_dealloc);
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(stats);
+
+static int mteswap_init(void)
+{
+	struct dentry *mteswap_dir;
+
+	mteswap_dir = debugfs_create_dir("mteswap", NULL);
+	debugfs_create_file("stats", 0444, mteswap_dir, NULL, &stats_fops);
+	return 0;
+}
+module_init(mteswap_init);
-- 
2.42.0.869.gea05f2083d-goog