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SubjectRe: [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;

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