Re: [PATCH v1 1/3] s390/uv: don't return 0 from make_hva_secure() if the operation was not successful

[David Hildenbrand <david@redhat.com>]

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