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@vger.kernel.org
Signed-off-by: David Hildenbrand <david@redhat.com>
---
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;
+ }
folio_put(folio);
return rc;
--
2.49.0On 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@vger.kernel.org
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> 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().
> + }
> folio_put(folio);
>
> return rc;
> --
> 2.49.0
--
Best Regards,
Yan, Zi
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@vger.kernel.org
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> 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@linux.ibm.com>
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
On 16 May 2025, at 17:20, David Hildenbrand wrote:
> 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@vger.kernel.org
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>> 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@linux.ibm.com>
> 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 :)
Got it. Acked-by: Zi Yan <ziy@nvidia.com>
--
Best Regards,
Yan, Zi
On 16.05.25 14:39, 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? Yes! I can reproduce it when allocating pages outside of the qemu process. $ echo force > /sys/kernel/mm/transparent_hugepage/shmem_enabled $ rm /dev/shm/vm-ram $ fallocate -l 4G /dev/shm/vm-ram $ /usr/libexec/qemu-kvm ... -object memory-backend-file,id=mem0,size=4g,share=on,mem-path=/dev/shm/vm-ram -M memory-backend=mem0 LOADPARM=[ ] Using virtio-blk. Using SCSI scheme. ......................................................................................................................... qemu-kvm: KVM PV command 4 (KVM_PV_VERIFY) failed: header rc 102 rrc 1a IOCTL rc: -22 Protected boot has failed: 0xa02 Guest crashed on cpu 0: disabled-wait PSW: 0x0002000080000000 0x0000000000004608 -- Cheers, David / dhildenb
© 2016 - 2025 Red Hat, Inc.