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

Catalin Marinas
2020-06-25T11:59:22Z

On Thu, Jun 25, 2020 at 12:37:40PM +0100, Steven Price wrote:
> On 24/06/2020 18:52, Catalin Marinas wrote:
> > 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