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: <aLlgesTc3ZIvgPg6@yzhao56-desk.sh.intel.com>
Date: Thu, 4 Sep 2025 17:48:42 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Binbin Wu <binbin.wu@...ux.intel.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>,
	<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 Thu, Sep 04, 2025 at 03:58:54PM +0800, Binbin Wu wrote:
> 
> 
> 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?
Hmm, either one (with or without curly braces) can pass the check of
"scripts/checkpatch.pl --strict".

> 
> >   	}
> > -	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?
What about:
The zap is limited to the range covered by a single folio.
As a S-EPT leaf entry can't cover a range larger than its backend folio size,
the zap can't cross two S-EPT leaf entries. So, no split is required.

> 
> > +	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
>