Re: [PATCH v5 22/25] arm64: mte: Enable swap of tagged pages

On Thu, Jun 25, 2020 at 12:37:40PM +0100, Steven Price wrote:

On 24/06/2020 18:52, Catalin Marinas wrote:

quoted

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 3e08aea56e7a..1712c504df15 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -10,6 +10,8 @@
  #include <linux/sched.h>
  #include <linux/sched/mm.h>
  #include <linux/string.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
  #include <linux/thread_info.h>
  #include <linux/uio.h>

@@ -18,15 +20,30 @@
  #include <asm/ptrace.h>
  #include <asm/sysreg.h>
+static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
+{
+	pte_t old_pte = READ_ONCE(*ptep);
+
+	if (check_swap && is_swap_pte(old_pte)) {
+		swp_entry_t entry = pte_to_swp_entry(old_pte);
+
+		if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
+			return;
+	}
+
+	mte_clear_page_tags(page_address(page));
+}
+
  void mte_sync_tags(pte_t *ptep, pte_t pte)
  {
  	struct page *page = pte_page(pte);
  	long i, nr_pages = compound_nr(page);
+	bool check_swap = nr_pages == 0;
  	/* if PG_mte_tagged is set, tags have already been initialised */
  	for (i = 0; i < nr_pages; i++, page++) {

This is broken - for check_swap to be true, nr_pages==0, which means we
never enter the loop and nothing happens...

Except I don't believe compound_nr() will return 0 - it's defined as:

  static inline unsigned long compound_nr(struct page *page)
  {
  	return 1UL << compound_order(page);
  }

Changing it to nr_pages==1 works for me.

Ah, I had it as compound_order() and changes to compound_nr() but left
the 0 check. Fixed locally.

Thanks.

-- 
Catalin

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