Re: [Patch v2] mm/rmap.c: split huge pmd when it really is

[Wei Yang <richardw.yang@linux.intel.com>]

On Mon, Dec 23, 2019 at 03:11:20PM -0800, Matthew Wilcox wrote:
>On Tue, Dec 24, 2019 at 06:28:56AM +0800, Wei Yang wrote:
>> When page is not NULL, function is called by try_to_unmap_one() with
>> TTU_SPLIT_HUGE_PMD set. There are two cases to call try_to_unmap_one()
>> with TTU_SPLIT_HUGE_PMD set:
>> 
>>   * unmap_page()
>>   * shrink_page_list()
>> 
>> In both case, the page passed to try_to_unmap_one() is PageHead() of the
>> THP. If this page's mapping address in process is not HPAGE_PMD_SIZE
>> aligned, this means the THP is not mapped as PMD THP in this process.
>> This could happen when we do mremap() a PMD size range to an un-aligned
>> address.
>> 
>> Currently, this case is handled by following check in __split_huge_pmd()
>> luckily.
>> 
>>   page != pmd_page(*pmd)
>> 
>> This patch checks the address to skip some work.
>
>The description here is confusing to me.
>

Sorry for the confusion.

Below is my understanding, if not correct or proper, just let me know :-)

According to current comment in __split_huge_pmd(), we check pmd_page with
page for migration case. While actually, this check also helps in the
following two cases when page already split-ed:

   * page just split-ed in place
   * page split-ed and moved to non-PMD aligned address

In both cases, pmd_page() is pointing to the PTE level page table. That's why
we don't split one already split-ed THP page.

If current code really intend to cover these two cases, sorry for my poor
understanding.

>> +	/*
>> +	 * When page is not NULL, function is called by try_to_unmap_one()
>> +	 * with TTU_SPLIT_HUGE_PMD set. There are two places set
>> +	 * TTU_SPLIT_HUGE_PMD
>> +	 *
>> +	 *     unmap_page()
>> +	 *     shrink_page_list()
>> +	 *
>> +	 * In both cases, the "page" here is the PageHead() of a THP.
>> +	 *
>> +	 * If the page is not a PMD mapped huge page, e.g. after mremap(), it
>> +	 * is not necessary to split it.
>> +	 */
>> +	if (page && !IS_ALIGNED(address, HPAGE_PMD_SIZE))
>> +		return;
>
>Repeating 75% of it as comments doesn't make it any less confusing.  And
>it feels like we're digging a pothole for someone to fall into later.
>Why not make it make sense ...
>
>	if (page && !IS_ALIGNED(address, page_size(page))
>		return;

Hmm... Use HPAGE_PMD_SIZE here wants to emphasize we want the address to be
PMD aligned. If just use page_size() here, may confuse the audience?

-- 
Wei Yang
Help you, Help me