linux-kernel - Re: [RFC PATCH v2 38/51] KVM: guest_memfd: Split allocator pages for guest

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Message-ID: <diqzo6v2huc4.fsf@ackerleytng-ctop.c.googlers.com>
Date: Thu, 05 Jun 2025 12:16:59 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
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Subject: Re: [RFC PATCH v2 38/51] KVM: guest_memfd: Split allocator pages for
 guest_memfd use

Binbin Wu <binbin.wu@...ux.intel.com> writes:

> On 5/15/2025 7:42 AM, Ackerley Tng wrote:
> [...]
>> +
>> +static inline int kvm_gmem_try_split_folio_in_filemap(struct inode *inode,
>> +						      struct folio *folio)
>> +{
>> +	size_t to_nr_pages;
>> +	void *priv;
>> +
>> +	if (!kvm_gmem_has_custom_allocator(inode))
>> +		return 0;
>> +
>> +	priv = kvm_gmem_allocator_private(inode);
>> +	to_nr_pages = kvm_gmem_allocator_ops(inode)->nr_pages_in_page(priv);
>> +
>> +	if (kvm_gmem_has_some_shared(inode, folio->index, to_nr_pages))
>
> What if a huge page whose attribute is shared?
>

This checks if there are any shared pages in the range [folio->index,
folio->index + to_nr_pages), so if the entire huge page is shared this
function should also return true.

folio->index is the start of the merged huge page, and to_nr_pages is
the number of pages in the merged huge page, so this should be querying
exactly the entire huge page.

Note to self: rename kvm_gmem_has_some_shared() to
kvm_gmem_has_any_shared() in the next revision.

Hope I answered your question! Let me know if I misunderstood your question.

>> +		return kvm_gmem_split_folio_in_filemap(inode, folio);
>> +
>> +	return 0;
>> +}
>> +
> [...]
>>   
>>   static int kvm_gmem_shareability_setup(struct maple_tree *mt, loff_t size, u64 flags)
>> @@ -563,11 +1005,16 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
>>   		return folio;
>>   
>>   	if (kvm_gmem_has_custom_allocator(inode)) {
>> -		void *p = kvm_gmem_allocator_private(inode);
>> +		size_t nr_pages;
>> +		void *p;
>>   
>> +		p = kvm_gmem_allocator_private(inode);
>>   		folio = kvm_gmem_allocator_ops(inode)->alloc_folio(p);
>>   		if (IS_ERR(folio))
>>   			return folio;
>> +
>> +		nr_pages = kvm_gmem_allocator_ops(inode)->nr_pages_in_folio(p);
>> +		index_floor = round_down(index, nr_pages);
>>   	} else {
>>   		gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
>>   
>> @@ -580,10 +1027,11 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
>>   			folio_put(folio);
>>   			return ERR_PTR(ret);
>>   		}
>> +
>> +		index_floor = index;
>>   	}
>>   	allocated_size = folio_size(folio);
>>   
>> -	index_floor = round_down(index, folio_nr_pages(folio));
>>   	ret = kvm_gmem_filemap_add_folio(inode->i_mapping, folio, index_floor);
>>   	if (ret) {
>>   		folio_put(folio);
>> @@ -600,6 +1048,13 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
>>   		return ERR_PTR(ret);
>>   	}
>>   
>> +	/* Leave just filemap's refcounts on folio. */
>> +	folio_put(folio);
>> +
>> +	ret = kvm_gmem_try_split_folio_in_filemap(inode, folio);
>
> When !CONFIG_KVM_GMEM_SHARED_MEM, kvm_gmem_try_split_folio_in_filemap() is
> undefined.
>

Will fix this in the next revision. Thanks!

>> +	if (ret)
>> +		goto err;
>> +
>>   	spin_lock(&inode->i_lock);
>>   	inode->i_blocks += allocated_size / 512;
>>   	spin_unlock(&inode->i_lock);
>>
> [...]