Re: [PATCH v8 05/13] KVM: guest_memfd: Add flag to remove from direct map

[Vlastimil Babka <vbabka@suse.cz>]

On 12/5/25 17:58, Kalyazin, Nikita wrote:
> +static int kvm_gmem_folio_zap_direct_map(struct folio *folio)
> +{
> +	int r = 0;
> +	unsigned long addr = (unsigned long) folio_address(folio);
> +	u64 gmem_flags = GMEM_I(folio_inode(folio))->flags;
> +
> +	if (kvm_gmem_folio_no_direct_map(folio) || !(gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP))
> +		goto out;
> +
> +	r = set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio),
> +					 false);
> +
> +	if (r)
> +		goto out;
> +
> +	folio->private = (void *) KVM_GMEM_FOLIO_NO_DIRECT_MAP;

With Dave's suggestion on patch 1/13 to have folio_zap_direct_map(), setting
this folio->private flag wouldn't be possible between the zap and tlb flush,
but it's not an issue to set it before the zap, right?

> +	flush_tlb_kernel_range(addr, addr + folio_size(folio));
> +
> +out:
> +	return r;
> +}
> +
> +static void kvm_gmem_folio_restore_direct_map(struct folio *folio)
> +{
> +	/*
> +	 * Direct map restoration cannot fail, as the only error condition
> +	 * for direct map manipulation is failure to allocate page tables
> +	 * when splitting huge pages, but this split would have already
> +	 * happened in set_direct_map_invalid_noflush() in kvm_gmem_folio_zap_direct_map().
> +	 * Thus set_direct_map_valid_noflush() here only updates prot bits.
> +	 */
> +	if (kvm_gmem_folio_no_direct_map(folio))
> +		set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio),
> +					 true);

I think you're missing here clearing KVM_GMEM_FOLIO_NO_DIRECT_MAP from
folio->private, which means if there's another
kvm_gmem_folio_zap_direct_map() call on it in the future, it will do nothing?

> +}
> +
>  static inline void kvm_gmem_mark_prepared(struct folio *folio)
>  {
>  	folio_mark_uptodate(folio);
> @@ -398,6 +444,7 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
>  	struct inode *inode = file_inode(vmf->vma->vm_file);
>  	struct folio *folio;
>  	vm_fault_t ret = VM_FAULT_LOCKED;
> +	int err;
>  
>  	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
>  		return VM_FAULT_SIGBUS;
> @@ -423,6 +470,12 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
>  		kvm_gmem_mark_prepared(folio);
>  	}
>  
> +	err = kvm_gmem_folio_zap_direct_map(folio);
> +	if (err) {
> +		ret = vmf_error(err);
> +		goto out_folio;
> +	}
> +
>  	vmf->page = folio_file_page(folio, vmf->pgoff);
>  
>  out_folio:
> @@ -533,6 +586,8 @@ static void kvm_gmem_free_folio(struct folio *folio)
>  	kvm_pfn_t pfn = page_to_pfn(page);
>  	int order = folio_order(folio);
>  
> +	kvm_gmem_folio_restore_direct_map(folio);
> +
>  	kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order));
>  }
>  
> @@ -596,6 +651,9 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
>  	/* Unmovable mappings are supposed to be marked unevictable as well. */
>  	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>  
> +	if (flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)
> +		mapping_set_no_direct_map(inode->i_mapping);
> +
>  	GMEM_I(inode)->flags = flags;
>  
>  	file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR, &kvm_gmem_fops);
> @@ -807,6 +865,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>  	if (!is_prepared)
>  		r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
>  
> +	kvm_gmem_folio_zap_direct_map(folio);
> +
>  	folio_unlock(folio);
>  
>  	if (!r)