[PATCH v3 4/7] arm64: mte: Lock a page for MTE tag initialisation

[Steven Price]

On 02/09/2022 17:58, Catalin Marinas wrote:
> On Fri, Sep 02, 2022 at 05:28:47PM +0100, Catalin Marinas wrote:
>> On Fri, Sep 02, 2022 at 03:47:33PM +0100, Steven Price wrote:
>>> On 10/08/2022 20:30, Peter Collingbourne wrote:
>>>> diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
>>>> index a78c1db23c68..cd5ad0936e16 100644
>>>> --- a/arch/arm64/mm/mteswap.c
>>>> +++ b/arch/arm64/mm/mteswap.c
>>>> @@ -53,6 +53,9 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
>>>>  	if (!tags)
>>>>  		return false;
>>>>  
>>>> +	/* racing tag restoring? */
>>>> +	if (!try_page_mte_tagging(page))
>>>> +		return false;
>>>>  	mte_restore_page_tags(page_address(page), tags);
>>>
>>> I feel like adding a "set_page_mte_tagged(page);" in here would avoid
>>> the need for the comments about mte_restore_tags() taking the lock.
>>
>> Good point. I think I blindly followed the set_bit() places but it makes
>> sense to move the bit setting to mte_restore_tags().
> 
> Something like this (and I need to do some more testing):

Exactly, that looks equivalent and is hopefully easier to follow.

Steve

> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index b956899467f0..be6560e1ff2b 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -25,7 +25,7 @@ unsigned long mte_copy_tags_to_user(void __user *to, void *from,
>  				    unsigned long n);
>  int mte_save_tags(struct page *page);
>  void mte_save_page_tags(const void *page_addr, void *tag_storage);
> -bool mte_restore_tags(swp_entry_t entry, struct page *page);
> +void mte_restore_tags(swp_entry_t entry, struct page *page);
>  void mte_restore_page_tags(void *page_addr, const void *tag_storage);
>  void mte_invalidate_tags(int type, pgoff_t offset);
>  void mte_invalidate_tags_area(int type);
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e6b82ad1e9e6..7d91379382fd 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1049,9 +1049,8 @@ static inline void arch_swap_invalidate_area(int type)
>  #define __HAVE_ARCH_SWAP_RESTORE
>  static inline void arch_swap_restore(swp_entry_t entry, struct folio *folio)
>  {
> -	/* mte_restore_tags() takes the PG_mte_lock */
> -	if (system_supports_mte() && mte_restore_tags(entry, &folio->page))
> -		set_page_mte_tagged(&folio->page);
> +	if (system_supports_mte())
> +		mte_restore_tags(entry, &folio->page);
>  }
>  
>  #endif /* CONFIG_ARM64_MTE */
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 634e089b5933..54ab6c4741db 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -41,11 +41,8 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
>  	if (check_swap && is_swap_pte(old_pte)) {
>  		swp_entry_t entry = pte_to_swp_entry(old_pte);
>  
> -		/* mte_restore_tags() takes the PG_mte_lock */
> -		if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
> -			set_page_mte_tagged(page);
> -			return;
> -		}
> +		if (!non_swap_entry(entry))
> +			mte_restore_tags(entry, page);
>  	}
>  
>  	if (!pte_is_tagged)
> diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
> index cd5ad0936e16..cd508ba80ab1 100644
> --- a/arch/arm64/mm/mteswap.c
> +++ b/arch/arm64/mm/mteswap.c
> @@ -46,19 +46,17 @@ int mte_save_tags(struct page *page)
>  	return 0;
>  }
>  
> -bool mte_restore_tags(swp_entry_t entry, struct page *page)
> +void mte_restore_tags(swp_entry_t entry, struct page *page)
>  {
>  	void *tags = xa_load(&mte_pages, entry.val);
>  
>  	if (!tags)
> -		return false;
> +		return;
>  
> -	/* racing tag restoring? */
> -	if (!try_page_mte_tagging(page))
> -		return false;
> -	mte_restore_page_tags(page_address(page), tags);
> -
> -	return true;
> +	if (try_page_mte_tagging(page)) {
> +		mte_restore_page_tags(page_address(page), tags);
> +		set_page_mte_tagged(page);
> +	}
>  }
>  
>  void mte_invalidate_tags(int type, pgoff_t offset)