Re: [PATCH] KVM: SEV: Pin SEV guest memory out of CMA area

[Ge Yang <yangge1116@126.com>]

在 2025/1/10 16:58, David Hildenbrand 写道:
> On 10.01.25 09:48, yangge1116@126.com wrote:
>> From: yangge <yangge1116@126.com>
> 
> I would suggest the title to be something like
> 
> "KVM: SEV: fix wrong pinning of pages"
> 
> Then you can describe that without FOLL_LONGTERM, the pages will not get 
> migrated out of MIGRATE_CMA/ZONE_MOVABLE, violating these mechanisms to 
> avoid fragmentation with unmovable pages, for example making CMA 
> allocations fail.
> 
> (CMA allocations failing is only one symptom of the missed usage of 
> FOLL_LONGTERM)
> 
>>
>> When pin_user_pages_fast() pins SEV guest memory without the
>> FOLL_LONGTERM flag, the pinned pages may inadvertently end up in the
>> CMA (Contiguous Memory Allocator) area. This can subsequently cause
>> cma_alloc() to fail in allocating these pages, due to the fact that
>> the pinned pages are not migratable.
>>
>> To address the aforementioned problem, we propose adding the
>> FOLL_LONGTERM flag to the pin_user_pages_fast() function. By doing
>> so, we ensure that the pages allocated will not occupy space within
>> the CMA area, thereby preventing potential allocation failures.
>>
>> Signed-off-by: yangge <yangge1116@126.com>
>> ---
>>   arch/x86/kvm/svm/sev.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index 943bd07..35d0714 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -630,6 +630,7 @@ static struct page **sev_pin_memory(struct kvm 
>> *kvm, unsigned long uaddr,
>>       unsigned long locked, lock_limit;
>>       struct page **pages;
>>       unsigned long first, last;
>> +    unsigned int flags = 0;
> 
> Why not set
> 
> unsigned int flags = FOLL_LONGTERM;
> 
>>       int ret;
>>       lockdep_assert_held(&kvm->lock);
>> @@ -662,8 +663,10 @@ static struct page **sev_pin_memory(struct kvm 
>> *kvm, unsigned long uaddr,
>>       if (!pages)
>>           return ERR_PTR(-ENOMEM);
>> +    flags = write ? FOLL_WRITE : 0;
> 
> 
> and here do
> 
> flags |= write ? FOLL_WRITE : 0;
> 
>> +
>>       /* Pin the user virtual address. */
>> -    npinned = pin_user_pages_fast(uaddr, npages, write ? FOLL_WRITE : 
>> 0, pages);
>> +    npinned = pin_user_pages_fast(uaddr, npages, flags | 
>> FOLL_LONGTERM, pages);
>>       if (npinned != npages) {
>>           pr_err("SEV: Failure locking %lu pages.\n", npages);
>>           ret = -ENOMEM;
> 
> 
Ok, thanks.