linux-kernel - Re: [PATCH v2 02/10] s390/uv: gmap_make_secure() cleanups for further changes

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Message-ID: <20240507172754.3f238ec7@p-imbrenda.boeblingen.de.ibm.com>
Date: Tue, 7 May 2024 17:27:54 +0200
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily
 Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle
 <svens@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Gerald
 Schaefer <gerald.schaefer@...ux.ibm.com>,
        Matthew Wilcox
 <willy@...radead.org>, Thomas Huth <thuth@...hat.com>
Subject: Re: [PATCH v2 02/10] s390/uv: gmap_make_secure() cleanups for
 further changes

On Fri, 12 Apr 2024 16:21:12 +0200
David Hildenbrand <david@...hat.com> wrote:

> Let's factor out handling of LRU cache draining and convert the if-else
> chain to a switch-case.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

it is a little ugly with the bool* parameter, but I can't think of
a better/nicer way to do it

Reviewed-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>

> ---
>  arch/s390/kernel/uv.c | 66 ++++++++++++++++++++++++++-----------------
>  1 file changed, 40 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index 016993e9eb72..25fe28d189df 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -266,6 +266,36 @@ static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_str
>  	return atomic_read(&mm->context.protected_count) > 1;
>  }
>  
> +/*
> + * Drain LRU caches: the local one on first invocation and the ones of all
> + * CPUs on successive invocations. Returns "true" on the first invocation.
> + */
> +static bool drain_lru(bool *drain_lru_called)
> +{
> +	/*
> +	 * If we have tried a local drain and the folio refcount
> +	 * still does not match our expected safe value, try with a
> +	 * system wide drain. This is needed if the pagevecs holding
> +	 * the page are on a different CPU.
> +	 */
> +	if (*drain_lru_called) {
> +		lru_add_drain_all();
> +		/* We give up here, don't retry immediately. */
> +		return false;
> +	}
> +	/*
> +	 * We are here if the folio refcount does not match the
> +	 * expected safe value. The main culprits are usually
> +	 * pagevecs. With lru_add_drain() we drain the pagevecs
> +	 * on the local CPU so that hopefully the refcount will
> +	 * reach the expected safe value.
> +	 */
> +	lru_add_drain();
> +	*drain_lru_called = true;
> +	/* The caller should try again immediately */
> +	return true;
> +}
> +
>  /*
>   * Requests the Ultravisor to make a page accessible to a guest.
>   * If it's brought in the first time, it will be cleared. If
> @@ -275,7 +305,7 @@ static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_str
>  int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>  {
>  	struct vm_area_struct *vma;
> -	bool local_drain = false;
> +	bool drain_lru_called = false;
>  	spinlock_t *ptelock;
>  	unsigned long uaddr;
>  	struct folio *folio;
> @@ -331,37 +361,21 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>  out:
>  	mmap_read_unlock(gmap->mm);
>  
> -	if (rc == -EAGAIN) {
> +	switch (rc) {
> +	case -EAGAIN:
>  		/*
>  		 * If we are here because the UVC returned busy or partial
>  		 * completion, this is just a useless check, but it is safe.
>  		 */
>  		folio_wait_writeback(folio);
>  		folio_put(folio);
> -	} else if (rc == -EBUSY) {
> -		/*
> -		 * If we have tried a local drain and the folio refcount
> -		 * still does not match our expected safe value, try with a
> -		 * system wide drain. This is needed if the pagevecs holding
> -		 * the page are on a different CPU.
> -		 */
> -		if (local_drain) {
> -			lru_add_drain_all();
> -			/* We give up here, and let the caller try again */
> -			return -EAGAIN;
> -		}
> -		/*
> -		 * We are here if the folio refcount does not match the
> -		 * expected safe value. The main culprits are usually
> -		 * pagevecs. With lru_add_drain() we drain the pagevecs
> -		 * on the local CPU so that hopefully the refcount will
> -		 * reach the expected safe value.
> -		 */
> -		lru_add_drain();
> -		local_drain = true;
> -		/* And now we try again immediately after draining */
> -		goto again;
> -	} else if (rc == -ENXIO) {
> +		return -EAGAIN;
> +	case -EBUSY:
> +		/* Additional folio references. */
> +		if (drain_lru(&drain_lru_called))
> +			goto again;
> +		return -EAGAIN;
> +	case -ENXIO:
>  		if (gmap_fault(gmap, gaddr, FAULT_FLAG_WRITE))
>  			return -EFAULT;
>  		return -EAGAIN;