Re: [PATCH v1 08/11] s390/uv: convert gmap_destroy_page() from follow_page() to folio_walk

[Claudio Imbrenda]

On Fri,  2 Aug 2024 17:55:21 +0200
David Hildenbrand <david@xxxxxxxxxx> wrote:

> Let's get rid of another follow_page() user and perform the UV calls
> under PTL -- which likely should be fine.
> 
> No need for an additional reference while holding the PTL:
> uv_destroy_folio() and uv_convert_from_secure_folio() raise the
> refcount, so any concurrent make_folio_secure() would see an unexpted
> reference and cannot set PG_arch_1 concurrently.
> 
> Do we really need a writable PTE? Likely yes, because the "destroy"
> part is, in comparison to the export, a destructive operation. So we'll
> keep the writability check for now.
> 
> We'll lose the secretmem check from follow_page(). Likely we don't care
> about that here.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>

> ---
>  arch/s390/kernel/uv.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index 35ed2aea8891..9646f773208a 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -14,6 +14,7 @@
>  #include <linux/memblock.h>
>  #include <linux/pagemap.h>
>  #include <linux/swap.h>
> +#include <linux/pagewalk.h>
>  #include <asm/facility.h>
>  #include <asm/sections.h>
>  #include <asm/uv.h>
> @@ -462,9 +463,9 @@ EXPORT_SYMBOL_GPL(gmap_convert_to_secure);
>  int gmap_destroy_page(struct gmap *gmap, unsigned long gaddr)
>  {
>  	struct vm_area_struct *vma;
> +	struct folio_walk fw;
>  	unsigned long uaddr;
>  	struct folio *folio;
> -	struct page *page;
>  	int rc;
>  
>  	rc = -EFAULT;
> @@ -483,11 +484,15 @@ int gmap_destroy_page(struct gmap *gmap, unsigned long gaddr)
>  		goto out;
>  
>  	rc = 0;
> -	/* we take an extra reference here */
> -	page = follow_page(vma, uaddr, FOLL_WRITE | FOLL_GET);
> -	if (IS_ERR_OR_NULL(page))
> +	folio = folio_walk_start(&fw, vma, uaddr, 0);
> +	if (!folio)
>  		goto out;
> -	folio = page_folio(page);
> +	/*
> +	 * See gmap_make_secure(): large folios cannot be secure. Small
> +	 * folio implies FW_LEVEL_PTE.
> +	 */
> +	if (folio_test_large(folio) || !pte_write(fw.pte))
> +		goto out_walk_end;
>  	rc = uv_destroy_folio(folio);
>  	/*
>  	 * Fault handlers can race; it is possible that two CPUs will fault
> @@ -500,7 +505,8 @@ int gmap_destroy_page(struct gmap *gmap, unsigned long gaddr)
>  	 */
>  	if (rc)
>  		rc = uv_convert_from_secure_folio(folio);
> -	folio_put(folio);
> +out_walk_end:
> +	folio_walk_end(&fw, vma);
>  out:
>  	mmap_read_unlock(gmap->mm);
>  	return rc;