linux-kernel - Re: [PATCH v1 1/3] s390/uv: don't return 0 from make_hva_secure() if the operation was not successful

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Message-ID: <60DDE99E-E09D-4BD4-AC58-569186E45660@nvidia.com>
Date: Fri, 16 May 2025 17:08:10 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
 kvm@...r.kernel.org, linux-mm@...ck.org,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Thomas Huth <thuth@...hat.com>,
 Matthew Wilcox <willy@...radead.org>,
 Sebastian Mitterle <smitterl@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH v1 1/3] s390/uv: don't return 0 from make_hva_secure() if
 the operation was not successful

On 16 May 2025, at 8:39, David Hildenbrand wrote:

> If s390_wiggle_split_folio() returns 0 because splitting a large folio
> succeeded, we will return 0 from make_hva_secure() even though a retry
> is required. Return -EAGAIN in that case.
>
> Otherwise, we'll return 0 from gmap_make_secure(), and consequently from
> unpack_one(). In kvm_s390_pv_unpack(), we assume that unpacking
> succeeded and skip unpacking this page. Later on, we run into issues
> and fail booting the VM.
>
> So far, this issue was only observed with follow-up patches where we
> split large pagecache XFS folios. Maybe it can also be triggered with
> shmem?
>
> We'll cleanup s390_wiggle_split_folio() a bit next, to also return 0
> if no split was required.
>
> Fixes: d8dfda5af0be ("KVM: s390: pv: fix race when making a page secure")
> Cc: stable@...r.kernel.org
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  arch/s390/kernel/uv.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index 9a5d5be8acf41..2cc3b599c7fe3 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -393,8 +393,11 @@ int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header
>  	folio_walk_end(&fw, vma);
>  	mmap_read_unlock(mm);
>
> -	if (rc == -E2BIG || rc == -EBUSY)
> +	if (rc == -E2BIG || rc == -EBUSY) {
>  		rc = s390_wiggle_split_folio(mm, folio, rc == -E2BIG);
> +		if (!rc)
> +			rc = -EAGAIN;

Why not just folio_put() then jump back to the beginning of the
function to do the retry? This could avoid going all the way back
to kvm_s390_unpack().

> +	}
>  	folio_put(folio);
>
>  	return rc;
> -- 
> 2.49.0


--
Best Regards,
Yan, Zi