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
