Re: [PATCH v1 09/11] s390/mm/fault: convert do_secure_storage_access() from follow_page() to folio_walk

[Claudio Imbrenda]

On Fri,  2 Aug 2024 17:55:22 +0200
David Hildenbrand <david@xxxxxxxxxx> wrote:

> Let's get rid of another follow_page() user and perform the conversion
> under PTL: Note that this is also what follow_page_pte() ends up doing.
> 
> Unfortunately we cannot currently optimize out the additional reference,
> because arch_make_folio_accessible() must be called with a raised
> refcount to protect against concurrent conversion to secure. We can just
> move the arch_make_folio_accessible() under the PTL, like
> follow_page_pte() would.
> 
> We'll effectively drop the "writable" check implied by FOLL_WRITE:
> follow_page_pte() would also not check that when calling
> arch_make_folio_accessible(), so there is no good reason for doing that
> here.
> 
> We'll lose the secretmem check from follow_page() as well, about which
> we shouldn't really care about.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>

> ---
>  arch/s390/mm/fault.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index 8e149ef5e89b..ad8b0d6b77ea 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -34,6 +34,7 @@
>  #include <linux/uaccess.h>
>  #include <linux/hugetlb.h>
>  #include <linux/kfence.h>
> +#include <linux/pagewalk.h>
>  #include <asm/asm-extable.h>
>  #include <asm/asm-offsets.h>
>  #include <asm/ptrace.h>
> @@ -492,9 +493,9 @@ void do_secure_storage_access(struct pt_regs *regs)
>  	union teid teid = { .val = regs->int_parm_long };
>  	unsigned long addr = get_fault_address(regs);
>  	struct vm_area_struct *vma;
> +	struct folio_walk fw;
>  	struct mm_struct *mm;
>  	struct folio *folio;
> -	struct page *page;
>  	struct gmap *gmap;
>  	int rc;
>  
> @@ -536,15 +537,18 @@ void do_secure_storage_access(struct pt_regs *regs)
>  		vma = find_vma(mm, addr);
>  		if (!vma)
>  			return handle_fault_error(regs, SEGV_MAPERR);
> -		page = follow_page(vma, addr, FOLL_WRITE | FOLL_GET);
> -		if (IS_ERR_OR_NULL(page)) {
> +		folio = folio_walk_start(&fw, vma, addr, 0);
> +		if (!folio) {
>  			mmap_read_unlock(mm);
>  			break;
>  		}
> -		folio = page_folio(page);
> -		if (arch_make_folio_accessible(folio))
> -			send_sig(SIGSEGV, current, 0);
> +		/* arch_make_folio_accessible() needs a raised refcount. */
> +		folio_get(folio);
> +		rc = arch_make_folio_accessible(folio);
>  		folio_put(folio);
> +		folio_walk_end(&fw, vma);
> +		if (rc)
> +			send_sig(SIGSEGV, current, 0);
>  		mmap_read_unlock(mm);
>  		break;
>  	case KERNEL_FAULT: