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;
