linux-kernel - Re: [RFC PATCH v2 33/51] KVM: guest_memfd: Allocate and truncate from custom allocator

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Message-ID: <diqza553hjil.fsf@ackerleytng-ctop.c.googlers.com>
Date: Wed, 16 Jul 2025 15:13:06 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
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Subject: Re: [RFC PATCH v2 33/51] KVM: guest_memfd: Allocate and truncate from
 custom allocator

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

> On 5/15/2025 7:42 AM, Ackerley Tng wrote:
> [...]
>>   
>>   	list_for_each_entry(gmem, gmem_list, entry)
>>   		kvm_gmem_invalidate_end(gmem, start, end);
>> @@ -776,6 +879,16 @@ static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
>>   
>>   	start = offset >> PAGE_SHIFT;
>>   	end = (offset + len) >> PAGE_SHIFT;
>> +	if (kvm_gmem_has_custom_allocator(inode)) {
>> +		size_t nr_pages;
>> +		void *p;
>> +
>> +		p = kvm_gmem_allocator_private(inode);
>> +		nr_pages = kvm_gmem_allocator_ops(inode)->nr_pages_in_folio(p);
>> +
>> +		start = round_down(start, nr_pages);
>> +		end = round_down(end, nr_pages);
> It's weird here.
> Should the end be round_up()?
>

Thanks, you're right.

I believe the current consensus is that fallocate() will only be
permitted for offset and lengths that are aligned not only to PAGE_SIZE
but to allocator page size.

In a future revision I'll check for allocator page size earlier on,
before this function will get called, so this rounding will probably go
away.

>> +	}
>>   
>>   	r = 0;
>>   	for (index = start; index < end; ) {
>>
> [...]