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

[Steven Price <steven.price@arm.com>]

On 24/06/2020 18:52, Catalin Marinas wrote:
> From: Steven Price <steven.price@arm.com>
> 
> When swapping pages out to disk it is necessary to save any tags that
> have been set, and restore when swapping back in. Make use of the new
> page flag (PG_ARCH_2, locally named PG_mte_tagged) to identify pages
> with tags. When swapping out these pages the tags are stored in memory
> and later restored when the pages are brought back in. Because shmem can
> swap pages back in without restoring the userspace PTE it is also
> necessary to add a hook for shmem.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> [catalin.marinas@arm.com: move function prototypes to mte.h]
> [catalin.marinas@arm.com: drop '_tags' from arch_swap_restore_tags()]
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Will Deacon <will@kernel.org>
> ---
> 
> Notes:
>      New in v4.
> 
[...]
> 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.

Steve