Re: [PATCH v2 13/13] mm/gup: move private gup FOLL_ flags to internal.h

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu, 26 Jan 2023 12:35:37 -0400
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Thu, Jan 26, 2023 at 04:39:02PM +0100, Claudio Imbrenda wrote:
> 
> > I can tell you that the original goal of that function is to make sure
> > that there are no extra references. in particular, we want to prevent
> > I/O of any kind to be ongoing while the page becomes secure. (the I/O
> > will fail and, depending on which device it was, the whole system might
> > end up in a rather unhappy state)  
> 
> Sure, but if there is concurrent IO you just try again right? It
> doesn't wait for refs to drop for instance.
> 
> So make the lock_page work the same way:

the more I look at this, the less I understand why I wrote the code
like that. I do have a comment, though

> 
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index 9f18a4af9c1319..847ee50b8672c6 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -193,20 +193,11 @@ static int expected_page_refs(struct page *page)
>  }
>  
>  static int make_secure_pte(pte_t *ptep, unsigned long addr,
> -			   struct page *exp_page, struct uv_cb_header *uvcb)
> +			   struct page *page, struct uv_cb_header *uvcb)
>  {
>  	pte_t entry = READ_ONCE(*ptep);
> -	struct page *page;
>  	int expected, cc = 0;
>  
> -	if (!pte_present(entry))
> -		return -ENXIO;
> -	if (pte_val(entry) & _PAGE_INVALID)
> -		return -ENXIO;
> -
> -	page = pte_page(entry);
> -	if (page != exp_page)
> -		return -ENXIO;
>  	if (PageWriteback(page))
>  		return -EAGAIN;
>  	expected = expected_page_refs(page);
> @@ -304,17 +295,25 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>  		goto out;
>  
>  	rc = -ENXIO;
> -	page = follow_page(vma, uaddr, FOLL_WRITE);
> -	if (IS_ERR_OR_NULL(page))
> -		goto out;
> -
> -	lock_page(page);
>  	ptep = get_locked_pte(gmap->mm, uaddr, &ptelock);
> +
> +	if (!pte_present(entry))
> +		goto out_unlock_pte;
> +	if (pte_val(entry) & _PAGE_INVALID)
> +		goto out_unlock_pte;

I guess we also need to make sure the page was writable?
FOLL_WRITE made sure of that

so I guess something like:

if (!pte_write(entry))
	goto out_unlock_pte;

> +	page = pte_page(entry);
> +
> +	if (!trylock_page(page)) {
> +		rc = -EAGAIN;
> +		goto out_unlock_pte;
> +	}
> +
>  	if (should_export_before_import(uvcb, gmap->mm))
>  		uv_convert_from_secure(page_to_phys(page));
>  	rc = make_secure_pte(ptep, uaddr, page, uvcb);
> -	pte_unmap_unlock(ptep, ptelock);
>  	unlock_page(page);
> +out_unlock_pte:
> +	pte_unmap_unlock(ptep, ptelock);
>  out:
>  	mmap_read_unlock(gmap->mm);
>