Re: [RFC PATCH 5/6] mm: truncate: split thp to a non-zero order if possible.

[Zi Yan <ziy@nvidia.com>]

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On 12 Nov 2020, at 17:08, Ralph Campbell wrote:

> On 11/11/20 12:40 PM, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> To minimize the number of pages after a truncation, when truncating a
>> THP, we do not need to split it all the way down to order-0. The THP has
>> at most three parts, the part before offset, the part to be truncated,
>> the part left at the end. Use the non-zero minimum of them to decide
>> what order we split the THP to.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   mm/truncate.c | 22 ++++++++++++++++++++--
>>   1 file changed, 20 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/truncate.c b/mm/truncate.c
>> index 20bd17538ec2..6d8e3c6115bc 100644
>> --- a/mm/truncate.c
>> +++ b/mm/truncate.c
>> @@ -237,7 +237,7 @@ int truncate_inode_page(struct address_space *mapping, struct page *page)
>>   bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t end)
>>   {
>>   	loff_t pos = page_offset(page);
>> -	unsigned int offset, length;
>> +	unsigned int offset, length, left, min_subpage_size = PAGE_SIZE;
>
> Maybe use "remaining" instead of "left" since I think of the latter as the length of the
> left side (offset).

Sure. Will change the name.

>
>>   	if (pos < start)
>>   		offset = start - pos;
>> @@ -248,6 +248,7 @@ bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t end)
>>   		length = length - offset;
>>   	else
>>   		length = end + 1 - pos - offset;
>> +	left = thp_size(page) - offset - length;
>>    	wait_on_page_writeback(page);
>>   	if (length == thp_size(page)) {
>> @@ -267,7 +268,24 @@ bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t end)
>>   		do_invalidatepage(page, offset, length);
>>   	if (!PageTransHuge(page))
>>   		return true;
>> -	return split_huge_page(page) == 0;
>> +
>> +	/*
>> +	 * find the non-zero minimum of offset, length, and left and use it to
>> +	 * decide the new order of the page after split
>> +	 */
>> +	if (offset && left)
>> +		min_subpage_size = min_t(unsigned int,
>> +					 min_t(unsigned int, offset, length),
>> +					 left);
>> +	else if (!offset)
>> +		min_subpage_size = min_t(unsigned int, length, left);
>> +	else /* !left */
>> +		min_subpage_size = min_t(unsigned int, length, offset);
>> +
>> +	min_subpage_size = max_t(unsigned int, PAGE_SIZE, min_subpage_size);
>> +
>> +	return split_huge_page_to_list_to_order(page, NULL,
>> +				ilog2(min_subpage_size/PAGE_SIZE)) == 0;
>>   }
>
> What if "min_subpage_size" is 1/2 the THP but offset isn't aligned to 1/2?
> Splitting the page in half wouldn't result in a page that could be freed
> but maybe splitting to 1/4 would (assuming the THP is at least 8x PAGE_SIZE).

Is it possible? The whole THP is divided into three parts, offset, length, and
remaining (renamed from left). If offset is not aligned to 1/2, it is either
greater than 1/2 or smaller than 1/2. If it is the former, length and remaining
will be smaller than 1/2, so min_subpage_size cannot be 1/2. If it is the latter,
min_subpage_size cannot be 1/2 either. Because min_subpage_size is the smallest
non-zero value of offset, length, and remaining. Let me know if I miss anything.

—
Best Regards,
Yan Zi

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