linux-kernel - Re: [RFC PATCH v2 17/23] KVM: guest_memfd: Split for punch hole and private-to-shared conversion

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Message-ID: <6b61cee4-0405-4967-afee-af934df34c5f@linux.intel.com>
Date: Thu, 4 Sep 2025 15:58:54 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, x86@...nel.org, rick.p.edgecombe@...el.com,
 dave.hansen@...el.com, kas@...nel.org, tabba@...gle.com,
 ackerleytng@...gle.com, quic_eberman@...cinc.com, michael.roth@....com,
 david@...hat.com, vannapurve@...gle.com, vbabka@...e.cz,
 thomas.lendacky@....com, pgonda@...gle.com, zhiquan1.li@...el.com,
 fan.du@...el.com, jun.miao@...el.com, ira.weiny@...el.com,
 isaku.yamahata@...el.com, xiaoyao.li@...el.com, chao.p.peng@...el.com
Subject: Re: [RFC PATCH v2 17/23] KVM: guest_memfd: Split for punch hole and
 private-to-shared conversion



On 8/7/2025 5:45 PM, Yan Zhao wrote:
[...]
>   
> @@ -514,6 +554,8 @@ static int kvm_gmem_convert_should_proceed(struct inode *inode,
>   					   struct conversion_work *work,
>   					   bool to_shared, pgoff_t *error_index)
>   {
> +	int ret = 0;
> +
>   	if (to_shared) {
>   		struct list_head *gmem_list;
>   		struct kvm_gmem *gmem;
> @@ -522,19 +564,24 @@ static int kvm_gmem_convert_should_proceed(struct inode *inode,
>   		work_end = work->start + work->nr_pages;
>   
>   		gmem_list = &inode->i_mapping->i_private_list;
> +		list_for_each_entry(gmem, gmem_list, entry) {
> +			ret = kvm_gmem_split_private(gmem, work->start, work_end);
> +			if (ret)
> +				return ret;
> +		}
>   		list_for_each_entry(gmem, gmem_list, entry)
> -			kvm_gmem_unmap_private(gmem, work->start, work_end);
> +			kvm_gmem_zap(gmem, work->start, work_end, KVM_FILTER_PRIVATE);
>   	} else {
>   		unmap_mapping_pages(inode->i_mapping, work->start,
>   				    work->nr_pages, false);
>   
>   		if (!kvm_gmem_has_safe_refcount(inode->i_mapping, work->start,
>   						work->nr_pages, error_index)) {
> -			return -EAGAIN;
> +			ret = -EAGAIN;
>   		}

Not from this patch.
When if statement breaks into two lines, are curly braces needed?


>   	}
>   
> -	return 0;
> +	return ret;
>   }
>   
[...]
> @@ -1906,8 +1926,14 @@ static int kvm_gmem_error_folio(struct address_space *mapping, struct folio *fol
>   	start = folio->index;
>   	end = start + folio_nr_pages(folio);
>   
> -	list_for_each_entry(gmem, gmem_list, entry)
> -		kvm_gmem_invalidate_begin_and_zap(gmem, start, end);
> +	/* The size of the SEPT will not exceed the size of the folio */
To me, the comment alone without the context doesn't give a direct expression that
split is not needed. If it's not too wordy, could you make it more informative?


> +	list_for_each_entry(gmem, gmem_list, entry) {
> +		enum kvm_gfn_range_filter filter;
> +
> +		kvm_gmem_invalidate_begin(gmem, start, end);
> +		filter = KVM_FILTER_PRIVATE | KVM_FILTER_SHARED;
> +		kvm_gmem_zap(gmem, start, end, filter);
> +	}
>   
>   	/*
>   	 * Do not truncate the range, what action is taken in response to the