[PATCH RFC 34/37] arm64: mte: Handle fatal signal in reserve_metadata_storage()

[Alexandru Elisei <alexandru.elisei@arm.com>]

As long as a fatal signal is pending, alloc_contig_range() will fail with
-EINTR. This makes it impossible for tag storage allocation to succeed, and
the page allocator will print an OOM splat.

The process is going to be killed, so return 0 (success) from
reserve_metadata_storage() to allow the page allocator to make progress.
set_pte_at() will map it with PAGE_METADATA_NONE and subsequent accesses
from different threads will trap until the signal is delivered.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
---
 arch/arm64/kernel/mte_tag_storage.c | 17 +++++++++++++++++
 arch/arm64/mm/fault.c               | 23 +++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/arch/arm64/kernel/mte_tag_storage.c b/arch/arm64/kernel/mte_tag_storage.c
index ce378f45f866..1ccbcc144979 100644
--- a/arch/arm64/kernel/mte_tag_storage.c
+++ b/arch/arm64/kernel/mte_tag_storage.c
@@ -556,6 +556,23 @@ int reserve_metadata_storage(struct page *page, int order, gfp_t gfp)
 				break;
 		}
 
+		/*
+		 * alloc_contig_range() returns -EINTR from
+		 * __alloc_contig_migrate_range() if a fatal signal is pending.
+		 * As long as the signal hasn't been handled, it is impossible
+		 * to reserve tag storage for any page. Treat it as an error,
+		 * but return 0 so the page allocator can make forward progress,
+		 * instead of printing an OOM splat.
+		 *
+		 * The tagged page with missing tag storage will be mapped with
+		 * PAGE_METADATA_NONE in set_pte_at(), and accesses until the
+		 * signal is delivered will cause a fault.
+		 */
+		if (ret == -EINTR) {
+			ret = 0;
+			goto out_error;
+		}
+
 		if (ret)
 			goto out_error;
 
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 7e2dcf5e3baf..64c5d77664c8 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -37,7 +37,9 @@
 #include <asm/debug-monitors.h>
 #include <asm/esr.h>
 #include <asm/kprobes.h>
+#include <asm/memory_metadata.h>
 #include <asm/mte.h>
+#include <asm/mte_tag_storage.h>
 #include <asm/processor.h>
 #include <asm/sysreg.h>
 #include <asm/system_misc.h>
@@ -936,10 +938,31 @@ void do_debug_exception(unsigned long addr_if_watchpoint, unsigned long esr,
 }
 NOKPROBE_SYMBOL(do_debug_exception);
 
+static void save_zero_page_tags(struct page *page)
+{
+	void *tags;
+
+	clear_page(page_address(page));
+
+	tags = kmalloc(MTE_PAGE_TAG_STORAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
+	if (WARN_ON(!tags))
+		return;
+
+	if (WARN_ON(mte_save_page_tags_by_pfn(page, tags)))
+		mte_free_tags_mem(tags);
+}
+
 void tag_clear_highpage(struct page *page)
 {
 	/* Tag storage pages cannot be tagged. */
 	WARN_ON_ONCE(is_migrate_metadata_page(page));
+
+	if (metadata_storage_enabled() &&
+	    unlikely(!page_tag_storage_reserved(page))) {
+		save_zero_page_tags(page);
+		return;
+	}
+
 	/* Newly allocated page, shouldn't have been tagged yet */
 	WARN_ON_ONCE(!try_page_mte_tagging(page));
 	mte_zero_clear_page_tags(page_address(page));
-- 
2.41.0