Patch "KVM: s390: pv: avoid stalls when making pages secure" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    KVM: s390: pv: avoid stalls when making pages secure

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-s390-pv-avoid-stalls-when-making-pages-secure.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 5ba85302f2e686742c608561e93f6750cd48ca32
Author: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Date:   Mon Sep 20 15:24:52 2021 +0200

    KVM: s390: pv: avoid stalls when making pages secure
    
    [ Upstream commit f0a1a0615a6ff6d38af2c65a522698fb4bb85df6 ]
    
    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.
    
    Due to the locks being held, it was not possible to simply replace
    uv_call with uv_call_sched. A more complex approach was
    needed, in which uv_call is replaced with __uv_call, which does not
    loop. When the UVC needs to be executed again, -EAGAIN is returned, and
    the caller (or its caller) will try again.
    
    When -EAGAIN is returned, the path is the same as when the page is in
    writeback (and the writeback check is also performed, which is
    harmless).
    
    Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests")
    Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
    Reviewed-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
    Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20210920132502.36111-5-imbrenda@xxxxxxxxxxxxx
    Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
    Stable-dep-of: 3f29f6537f54 ("s390/uv: Don't call folio_wait_writeback() without a folio reference")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index f95ccbd396925..09b80d371409b 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -165,7 +165,7 @@ 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;
@@ -181,12 +181,25 @@ 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;
 }
 
 /*
@@ -239,6 +252,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 	mmap_read_unlock(gmap->mm);
 
 	if (rc == -EAGAIN) {
+		/*
+		 * If we are here because the UVC returned busy or partial
+		 * completion, this is just a useless check, but it is safe.
+		 */
 		wait_on_page_writeback(page);
 	} else if (rc == -EBUSY) {
 		/*
diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index 458b42b50b8cb..79e6f016c22fc 100644
--- a/arch/s390/kvm/intercept.c
+++ b/arch/s390/kvm/intercept.c
@@ -537,6 +537,11 @@ static int handle_pv_uvc(struct kvm_vcpu *vcpu)
 	 */
 	if (rc == -EINVAL)
 		return 0;
+	/*
+	 * If we got -EAGAIN here, we simply return it. It will eventually
+	 * get propagated all the way to userspace, which should then try
+	 * again.
+	 */
 	return rc;
 }