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.
