On 16.05.25 23:08, Zi Yan wrote:
On 16 May 2025, at 8:39, David Hildenbrand wrote:
If s390_wiggle_split_folio() returns 0 because splitting a large folio
succeeded, we will return 0 from make_hva_secure() even though a retry
is required. Return -EAGAIN in that case.
Otherwise, we'll return 0 from gmap_make_secure(), and consequently from
unpack_one(). In kvm_s390_pv_unpack(), we assume that unpacking
succeeded and skip unpacking this page. Later on, we run into issues
and fail booting the VM.
So far, this issue was only observed with follow-up patches where we
split large pagecache XFS folios. Maybe it can also be triggered with
shmem?
We'll cleanup s390_wiggle_split_folio() a bit next, to also return 0
if no split was required.
Fixes: d8dfda5af0be ("KVM: s390: pv: fix race when making a page secure")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
arch/s390/kernel/uv.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index 9a5d5be8acf41..2cc3b599c7fe3 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -393,8 +393,11 @@ int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header
folio_walk_end(&fw, vma);
mmap_read_unlock(mm);
- if (rc == -E2BIG || rc == -EBUSY)
+ if (rc == -E2BIG || rc == -EBUSY) {
rc = s390_wiggle_split_folio(mm, folio, rc == -E2BIG);
+ if (!rc)
+ rc = -EAGAIN;
Why not just folio_put() then jump back to the beginning of the
function to do the retry? This could avoid going all the way back
to kvm_s390_unpack().
Hi, thanks for the review.
We had a pretty optimized version with such tricks before Claudio
refactored it in:
commit 5cbe24350b7d8ef6d466a37d56b07ae643c622ca
Author: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Date: Thu Jan 23 15:46:17 2025 +0100
KVM: s390: move pv gmap functions into kvm
In particular, one relevant hunk was:
- switch (rc) {
- case -E2BIG:
- folio_lock(folio);
- rc = split_folio(folio);
- folio_unlock(folio);
- folio_put(folio);
-
- switch (rc) {
- case 0:
- /* Splitting succeeded, try again immediately. */
- goto again;
- case -EAGAIN:
- /* Additional folio references. */
- if (drain_lru(&drain_lru_called))
- goto again;
- return -EAGAIN;
Claudio probably had a good reason to rewrite the code -- and I hope
we'll be able to rip all of that out soon, so ...
... minimal changes until then :)
--
Cheers,
David / dhildenb
