Re: [PATCH v2 03/10] s390/uv: split large folios in gmap_make_secure()

[Claudio Imbrenda]

On Fri, 12 Apr 2024 16:21:13 +0200
David Hildenbrand <david@xxxxxxxxxx> wrote:

> While s390x makes sure to never have PMD-mapped THP in processes that use
> KVM -- by remapping them using PTEs in
> thp_split_walk_pmd_entry()->split_huge_pmd() -- there is still the
> possibility of having PTE-mapped THPs (large folios) mapped into guest
> memory.
> 
> This would happen if user space allocates memory before calling
> KVM_CREATE_VM (which would call s390_enable_sie()). With upstream QEMU,
> this currently doesn't happen, because guest memory is setup and
> condiitonally preallocated after KVM_CREATE_VM.

*conditionally

> 
> Could it happen with shmem/file-backed memory when another process
> allocated memory in the pagecache? Likely, although currently not a
> common setup.
> 
> Trying to split any PTE-mapped large folios sounds like the right and
> future-proof thing to do here. So let's call split_folio() and handle the
> return values accordingly.

Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>

> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  arch/s390/kernel/uv.c | 31 +++++++++++++++++++++++++------
>  1 file changed, 25 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index 25fe28d189df..3c6d86e3e828 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -338,11 +338,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>  		goto out;
>  	if (pte_present(*ptep) && !(pte_val(*ptep) & _PAGE_INVALID) && pte_write(*ptep)) {
>  		folio = page_folio(pte_page(*ptep));
> -		rc = -EINVAL;
> -		if (folio_test_large(folio))
> -			goto unlock;
>  		rc = -EAGAIN;
> -		if (folio_trylock(folio)) {
> +		if (folio_test_large(folio)) {
> +			rc = -E2BIG;
> +		} else if (folio_trylock(folio)) {
>  			if (should_export_before_import(uvcb, gmap->mm))
>  				uv_convert_from_secure(PFN_PHYS(folio_pfn(folio)));
>  			rc = make_folio_secure(folio, uvcb);
> @@ -353,15 +352,35 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>  		 * Once we drop the PTL, the folio may get unmapped and
>  		 * freed immediately. We need a temporary reference.
>  		 */
> -		if (rc == -EAGAIN)
> +		if (rc == -EAGAIN || rc == -E2BIG)
>  			folio_get(folio);
>  	}
> -unlock:
>  	pte_unmap_unlock(ptep, ptelock);
>  out:
>  	mmap_read_unlock(gmap->mm);
>  
>  	switch (rc) {
> +	case -E2BIG:
> +		folio_lock(folio);
> +		rc = split_folio(folio);
> +		folio_unlock(folio);
> +		folio_put(folio);
> +
> +		switch (rc) {
> +		case 0:
> +			/* Splitting succeeded, try again immediately. */
> +			goto again;
> +		case -EAGAIN:
> +			/* Additional folio references. */
> +			if (drain_lru(&drain_lru_called))
> +				goto again;
> +			return -EAGAIN;
> +		case -EBUSY:
> +			/* Unexpected race. */
> +			return -EAGAIN;
> +		}
> +		WARN_ON_ONCE(1);
> +		return -ENXIO;
>  	case -EAGAIN:
>  		/*
>  		 * If we are here because the UVC returned busy or partial