linux-kernel - Re: [PATCH RFC v3 1/2] KVM: guest_memfd: Convert .free_folio() to .release

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Message-ID: <c650066d-18c8-4711-ae22-3c6c660c713e@redhat.com>
Date: Fri, 15 Nov 2024 11:58:28 +0100
From: David Hildenbrand <david@...hat.com>
To: Elliot Berman <quic_eberman@...cinc.com>,
 Paolo Bonzini <pbonzini@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Sean Christopherson <seanjc@...gle.com>, Fuad Tabba <tabba@...gle.com>,
 Ackerley Tng <ackerleytng@...gle.com>, Mike Rapoport <rppt@...nel.org>,
 "H. Peter Anvin" <hpa@...or.com>
Cc: James Gowans <jgowans@...zon.com>, linux-fsdevel@...r.kernel.org,
 kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
 linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH RFC v3 1/2] KVM: guest_memfd: Convert .free_folio() to
 .release_folio()

On 13.11.24 23:34, Elliot Berman wrote:
> When guest_memfd becomes a library, a callback will need to be made to
> the owner (KVM SEV) to transition pages back to hypervisor-owned/shared
> state. This is currently being done as part of .free_folio() address
> space op, but this callback shouldn't assume that the mapping still
> exists. guest_memfd library will need the mapping to still exist to look
> up its operations table.

I assume you mean, that the mapping is no longer set for the folio (it 
sure still exists, because we are getting a callback from it :) )?

Staring at filemap_remove_folio(), this is exactly what happens:

We remember folio->mapping, call __filemap_remove_folio(), and then call 
filemap_free_folio() where we zap folio->mapping via page_cache_delete().

Maybe it's easier+cleaner to also forward the mapping to the 
free_folio() callback, just like we do with filemap_free_folio()? Would 
that help?

CCing Willy if that would be reasonable extension of the free_folio 
callback.

> 
> .release_folio() and .invalidate_folio() address space ops can serve the
> same purpose here. The key difference between release_folio() and
> free_folio() is whether the mapping is still valid at time of the
> callback. This approach was discussed in the link in the footer, but not
> taken because free_folio() was easier to implement.
> 
> Link: https://lore.kernel.org/kvm/20231016115028.996656-1-michael.roth@amd.com/
> Signed-off-by: Elliot Berman <quic_eberman@...cinc.com>
> ---
>   virt/kvm/guest_memfd.c | 19 ++++++++++++++++---
>   1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 47a9f68f7b247f4cba0c958b4c7cd9458e7c46b4..13f83ad8a4c26ba82aca4f2684f22044abb4bc19 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -358,22 +358,35 @@ static int kvm_gmem_error_folio(struct address_space *mapping, struct folio *fol
>   }
>   
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE
> -static void kvm_gmem_free_folio(struct folio *folio)
> +static bool kvm_gmem_release_folio(struct folio *folio, gfp_t gfp)
>   {
>   	struct page *page = folio_page(folio, 0);
>   	kvm_pfn_t pfn = page_to_pfn(page);
>   	int order = folio_order(folio);
>   
>   	kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order));
> +
> +	return true;
> +}
> +
> +static void kvm_gmem_invalidate_folio(struct folio *folio, size_t offset,
> +				      size_t len)
> +{
> +	WARN_ON_ONCE(offset != 0);
> +	WARN_ON_ONCE(len != folio_size(folio));
> +
> +	if (offset == 0 && len == folio_size(folio))
> +		filemap_release_folio(folio, 0);
>   }
>   #endif
>   
>   static const struct address_space_operations kvm_gmem_aops = {
>   	.dirty_folio = noop_dirty_folio,
> -	.migrate_folio	= kvm_gmem_migrate_folio,
> +	.migrate_folio = kvm_gmem_migrate_folio,
>   	.error_remove_folio = kvm_gmem_error_folio,
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE
> -	.free_folio = kvm_gmem_free_folio,
> +	.release_folio = kvm_gmem_release_folio,
> +	.invalidate_folio = kvm_gmem_invalidate_folio,
>   #endif
>   };
>   
> 

-- 
Cheers,

David / dhildenb