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

Alexandru Elisei
2023-08-23T13:19:33Z

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