On Tue, Feb 11, 2020 at 08:18:22PM -0800, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> > > If THP is disabled, find_subpage can become a no-op. > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > --- > include/linux/pagemap.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h > index 75bdfec49710..0842622cca90 100644 > --- a/include/linux/pagemap.h > +++ b/include/linux/pagemap.h > @@ -340,7 +340,7 @@ static inline struct page *find_subpage(struct page *page, pgoff_t offset) > > VM_BUG_ON_PAGE(PageTail(page), page); > > - return page + (offset & (compound_nr(page) - 1)); > + return page + (offset & (hpage_nr_pages(page) - 1)); > } So just above the quoted code is a if (PageHuge(page)) return page; So how can we get into a compound page that is not a huge page, but only if THP is enabled? (Yes, maybe I'm a little rusty on VM internals). Can you add comments describing the use case of this function and why it does all these checks? It looks like black magic to me.
