[PATCH v11-mte 6/7] 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 attempt 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

(statistics collection is introduced in the following patch)

Signed-off-by: Alexander Potapenko <glider at google.com>
Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>

---
 v10-mte:
  - added Catalin's Reviewed-by:

 v9:
  - as requested by Yury Norov, split off statistics collection into a
    separate patch
  - minor fixes

 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 | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index a31833e3ddc54..70f5c8ecd640d 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -6,6 +6,8 @@
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <asm/mte.h>
+#include <asm/mtecomp.h>
+#include "mtecomp.h"
 
 static DEFINE_XARRAY(mte_pages);
 
@@ -17,12 +19,13 @@ void *mte_allocate_tag_storage(void)
 
 void mte_free_tag_storage(char *storage)
 {
-	kfree(storage);
+	if (!mte_is_compressed(storage))
+		kfree(storage);
 }
 
 int mte_save_tags(struct page *page)
 {
-	void *tag_storage, *ret;
+	void *tag_storage, *compressed_storage, *ret;
 
 	if (!page_mte_tagged(page))
 		return 0;
@@ -32,6 +35,11 @@ int mte_save_tags(struct page *page)
 		return -ENOMEM;
 
 	mte_save_page_tags(page_address(page), tag_storage);
+	compressed_storage = mte_compress(tag_storage);
+	if (compressed_storage) {
+		mte_free_tag_storage(tag_storage);
+		tag_storage = compressed_storage;
+	}
 
 	/* lookup the swap entry.val from the page */
 	ret = xa_store(&mte_pages, page_swap_entry(page).val, tag_storage,
@@ -50,13 +58,20 @@ 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);
 	}
 }
 
-- 
2.43.0.472.g3155946c3a-goog