linux-kernel - Re: [PATCH v1 2/3] s390/uv: always return 0 from s390_wiggle_split

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Message-ID: <E1DF5F15-1211-4E4A-AD1B-BCD2396DEFB6@nvidia.com>
Date: Fri, 16 May 2025 20:08:53 -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>
Subject: Re: [PATCH v1 2/3] s390/uv: always return 0 from
 s390_wiggle_split_folio() if successful

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

> Let's consistently return 0 if the operation was successful, and just
> detect ourselves whether splitting is required -- folio_test_large() is
> a cheap operation.
>
> Update the documentation.
>
> Should we simply always return -EAGAIN instead of 0, so we don't have
> to handle it in the caller? Not sure, staring at the documentation, this
> way looks a bit cleaner.
>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  arch/s390/kernel/uv.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index 2cc3b599c7fe3..f6ddb2b54032e 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -324,34 +324,36 @@ static int make_folio_secure(struct mm_struct *mm, struct folio *folio, struct u
>  }
>
>  /**
> - * s390_wiggle_split_folio() - try to drain extra references to a folio and optionally split.
> + * s390_wiggle_split_folio() - try to drain extra references to a folio and
> + *			       split the folio if it is large.
>   * @mm:    the mm containing the folio to work on
>   * @folio: the folio
> - * @split: whether to split a large folio
>   *
>   * Context: Must be called while holding an extra reference to the folio;
>   *          the mm lock should not be held.
> - * Return: 0 if the folio was split successfully;
> - *         -EAGAIN if the folio was not split successfully but another attempt
> - *                 can be made, or if @split was set to false;
> - *         -EINVAL in case of other errors. See split_folio().
> + * Return: 0 if the operation was successful;
> + *	   -EAGAIN if splitting the large folio was not successful,
> + *		   but another attempt can be made;
> + *	   -EINVAL in case of other folio splitting errors. See split_folio().
>   */
> -static int s390_wiggle_split_folio(struct mm_struct *mm, struct folio *folio, bool split)
> +static int s390_wiggle_split_folio(struct mm_struct *mm, struct folio *folio)
>  {
>  	int rc;
>
>  	lockdep_assert_not_held(&mm->mmap_lock);
>  	folio_wait_writeback(folio);
>  	lru_add_drain_all();
> -	if (split) {
> +
> +	if (folio_test_large(folio)) {
>  		folio_lock(folio);
>  		rc = split_folio(folio);
>  		folio_unlock(folio);
>
>  		if (rc != -EBUSY)
>  			return rc;
> +		return -EAGAIN;
>  	}
> -	return -EAGAIN;
> +	return 0;
>  }

I can see how this function is written to service as two purposes,
trying to get rid of pcp ref of a folio and split a folio (to avoid
the extra pcp ref from failing split, lru_add_drain_all() is
called before split). Hope it will be refactored later.

>
>  int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header *uvcb)
> @@ -394,7 +396,7 @@ int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header
>  	mmap_read_unlock(mm);
>
>  	if (rc == -E2BIG || rc == -EBUSY) {
> -		rc = s390_wiggle_split_folio(mm, folio, rc == -E2BIG);
> +		rc = s390_wiggle_split_folio(mm, folio);
>  		if (!rc)
>  			rc = -EAGAIN;
>  	}
> -- 
> 2.49.0

The changes look good to me. Acked-by: Zi Yan <ziy@...dia.com>

--
Best Regards,
Yan, Zi