[PATCH RFC v2 24/27] arm64: mte: Handle fatal signal in reserve_tag_storage()

Alexandru Elisei
2023-11-19T17:00:24Z

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