Re: [RFC PATCH 15/39] KVM: guest_memfd: hugetlb: allocate and truncate from hugetlb

[Elliot Berman <quic_eberman@quicinc.com>]

On Tue, Sep 10, 2024 at 11:43:46PM +0000, Ackerley Tng wrote:
> If HugeTLB is requested at guest_memfd creation time, HugeTLB pages
> will be used to back guest_memfd.
> 
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> ---
>  virt/kvm/guest_memfd.c | 252 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 239 insertions(+), 13 deletions(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 31e1115273e1..2e6f12e2bac8 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -8,6 +8,8 @@
>  #include <linux/pseudo_fs.h>
>  #include <linux/pagemap.h>
>  #include <linux/anon_inodes.h>
> +#include <linux/memcontrol.h>
> +#include <linux/mempolicy.h>
>  
>  #include "kvm_mm.h"
>  
> @@ -29,6 +31,13 @@ static struct kvm_gmem_hugetlb *kvm_gmem_hgmem(struct inode *inode)
>  	return inode->i_mapping->i_private_data;
>  }
>  
> +static bool is_kvm_gmem_hugetlb(struct inode *inode)
> +{
> +	u64 flags = (u64)inode->i_private;
> +
> +	return flags & KVM_GUEST_MEMFD_HUGETLB;
> +}
> +
>  /**
>   * folio_file_pfn - like folio_file_page, but return a pfn.
>   * @folio: The folio which contains this index.
> @@ -58,6 +67,9 @@ static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slo
>  	return 0;
>  }
>  
> +/**
> + * Use the uptodate flag to indicate that the folio is prepared for KVM's usage.
> + */
>  static inline void kvm_gmem_mark_prepared(struct folio *folio)
>  {
>  	folio_mark_uptodate(folio);
> @@ -72,13 +84,18 @@ static inline void kvm_gmem_mark_prepared(struct folio *folio)
>  static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
>  				  gfn_t gfn, struct folio *folio)
>  {
> -	unsigned long nr_pages, i;
>  	pgoff_t index;
>  	int r;
>  
> -	nr_pages = folio_nr_pages(folio);
> -	for (i = 0; i < nr_pages; i++)
> -		clear_highpage(folio_page(folio, i));
> +	if (folio_test_hugetlb(folio)) {
> +		folio_zero_user(folio, folio->index << PAGE_SHIFT);

Is (folio->index << PAGE_SHIFT) the right address hint to provide?
I don't think we can say the folio will be mapped at this address since
this value is an offset into the file.  In most cases, I believe it
won't be mapped anywhere since we just allocated it.

Thanks,
Elliot