On Thu, 29 Jul 2021 12:49:03 +0200
Cornelia Huck [off-list ref] wrote:

On Wed, Jul 28 2021, Claudio Imbrenda [off-list ref] 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.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kernel/uv.c | 11 ++++++++---
 arch/s390/kvm/pv.c    |  2 +-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index aeb0a15bcbb7..fd0faa51c1bb 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c

@@ -196,11 +196,16 @@ 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);
+	rc = __uv_call(0, (u64)uvcb);
 	page_ref_unfreeze(page, expected);
-	/* Return -ENXIO if the page was not mapped, -EINVAL
otherwise */
-	if (rc)
+	/*
+	 * Return -ENXIO if the page was not mapped, -EINVAL for
other errors.
+	 * If busy or partially completed, return -EAGAIN.
+	 */
+	if (rc == 1)
 		rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
+	else if (rc > 1)
+		rc = -EAGAIN;
 	return rc;
 }  

Possibly dumb question: when does the call return > 1?

this is exactly what Janosch meant :)

the next version will have #defines for the 4 possible CC values.

in short:
0 OK
1 error
2 busy (nothing done, try again)
3 partial (something done but not all, try again)

gmap_make_secure() will do a wait_on_page_writeback() for -EAGAIN, is
that always the right thing to do?

it's the easiest way to get to a place where we will be able to
reschedule if needed.

wait_on_page_writeback will probably do nothing in that case because
the page is not in writeback.

(a few minutes later)

actually I have checked, it seems that the -EAGAIN gets eventually
propagated to places where it's not checked properly!

this will need some more fixing