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

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Date:   Mon, 23 Dec 2019 15:11:20 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com
Subject: Re: [Patch v2] mm/rmap.c: split huge pmd when it really is

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.

> +	/*
> +	 * 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;