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 Help you, Help me
