On Fri, 6 Aug 2021 09:30:04 +0200
David Hildenbrand [off-list ref] wrote:

On 04.08.21 17:40, Claudio Imbrenda wrote:

quoted

Improve make_secure_pte to avoid stalls when the system is heavily
overcommitted. This was especially problematic in
kvm_s390_pv_unpack, because of the loop over all pages that needed
unpacking.

Also fix kvm_s390_pv_init_vm to avoid stalls when the system is
heavily overcommitted.  

I suggest splitting this change into a separate patch and adding a
bit more meat to the description why using the other variant is
possible in the called context. I was kind of surprise to find that
change buried in this patch.

Then, you can give both patches a more descriptive patch subject.

fair enough, I'll split them

quoted

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management
functions for protected KVM guests") ---
  arch/s390/kernel/uv.c | 29 +++++++++++++++++++++++------
  arch/s390/kvm/pv.c    |  2 +-
  2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index aeb0a15bcbb7..68a8fbafcb9c 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c

@@ -180,7 +180,7 @@ static int make_secure_pte(pte_t *ptep,

unsigned long addr, {
  	pte_t entry = READ_ONCE(*ptep);
  	struct page *page;
-	int expected, rc = 0;
+	int expected, cc = 0;
  
  	if (!pte_present(entry))
  		return -ENXIO;

@@ -196,12 +196,25 @@ static int make_secure_pte(pte_t *ptep,

unsigned long addr, if (!page_ref_freeze(page, expected))
  		return -EBUSY;
  	set_bit(PG_arch_1, &page->flags);
-	rc = uv_call(0, (u64)uvcb);
+	/*
+	 * If the UVC does not succeed or fail immediately, we
don't want to
+	 * loop for long, or we might get stall notifications.
+	 * On the other hand, this is a complex scenario and we
are holding a lot of
+	 * locks, so we can't easily sleep and reschedule. We try
only once,
+	 * and if the UVC returned busy or partial completion, we
return
+	 * -EAGAIN and we let the callers deal with it.
+	 */
+	cc = __uv_call(0, (u64)uvcb);
  	page_ref_unfreeze(page, expected);
-	/* Return -ENXIO if the page was not mapped, -EINVAL
otherwise */
-	if (rc)
-		rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
-	return rc;
+	/*
+	 * Return -ENXIO if the page was not mapped, -EINVAL for
other errors.
+	 * If busy or partially completed, return -EAGAIN.
+	 */
+	if (cc == UVC_CC_OK)
+		return 0;
+	else if (cc == UVC_CC_BUSY || cc == UVC_CC_PARTIAL)
+		return -EAGAIN;
+	return uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
  }  

That looks conceptually like the right thing to me.