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

[Wei Yang]

On Fri, Dec 27, 2019 at 07:13:46AM -0800, Matthew Wilcox wrote:
>On Tue, Dec 24, 2019 at 09:56:02AM +0800, Wei Yang wrote:
>> 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?
>
>I'm OK with using HPAGE_PMD_SIZE here.  I was trying to future-proof
>this function for supporting 64kB pages with a 4kB page size on ARM,
>but this function will need changes for that anyway, so I'm OK with
>your suggestion.

Thanks for your comments. :-)

-- 
Wei Yang
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