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

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Message-ID: <26fe43db-b122-40e0-a05a-81b11b89ef46@redhat.com>
Date: Fri, 10 Jan 2025 09:58:02 +0100
From: David Hildenbrand <david@...hat.com>
To: yangge1116@....com, pbonzini@...hat.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, michael.roth@....com,
 21cnbao@...il.com, baolin.wang@...ux.alibaba.com, liuzixing@...on.cn
Subject: Re: [PATCH] KVM: SEV: Pin SEV guest memory out of CMA area

On 10.01.25 09:48, yangge1116@....com wrote:
> From: yangge <yangge1116@....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@....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;


-- 
Cheers,

David / dhildenb