[PATCH RFC v2 24/27] arm64: mte: Handle fatal signal in reserve_tag_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_tag_storage() to allow the page allocator to make progress.
set_pte_at() will map it with PAGE_FAULT_ON_ACCESS and subsequent accesses
from different threads will cause a fault 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               |  5 +++++
 2 files changed, 22 insertions(+)

diff --git a/arch/arm64/kernel/mte_tag_storage.c b/arch/arm64/kernel/mte_tag_storage.c
index 6b11bb408b51..602fdc70db1c 100644
--- a/arch/arm64/kernel/mte_tag_storage.c
+++ b/arch/arm64/kernel/mte_tag_storage.c
@@ -572,6 +572,23 @@ int reserve_tag_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. Stop trying to reserve
+		 * tag storage, 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_FAULT_ON_ACCESS in set_pte_at(), which means 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 964c5ae161a3..fdc98c5828bf 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -950,6 +950,11 @@ gfp_t arch_calc_vma_gfp(struct vm_area_struct *vma, gfp_t gfp)
 
 void tag_clear_highpage(struct page *page)
 {
+	if (tag_storage_enabled() && unlikely(!page_tag_storage_reserved(page))) {
+		clear_page(page_address(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));
-- 
2.42.1