Re: page cache: Store only head pages in i_pages

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Apr 04, 2019 at 09:10:10AM -0400, Qian Cai wrote:
> On Mon, 2019-04-01 at 12:27 +0300, Kirill A. Shutemov wrote:
> > What about patch like this? (completely untested)
> > 
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index f939e004c5d1..e3b9bf843dcb 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -335,12 +335,12 @@ static inline struct page *grab_cache_page_nowait(struct
> > address_space *mapping,
> >  
> >  static inline struct page *find_subpage(struct page *page, pgoff_t offset)
> >  {
> > -	unsigned long index = page_index(page);
> > +	unsigned long mask;
> >  
> >  	VM_BUG_ON_PAGE(PageTail(page), page);
> > -	VM_BUG_ON_PAGE(index > offset, page);
> > -	VM_BUG_ON_PAGE(index + (1 << compound_order(page)) <= offset, page);
> > -	return page - index + offset;
> > +
> > +	mask = (1UL << compound_order(page)) - 1;
> > +	return page + (offset & mask);
> >  }
> >  
> >  struct page *find_get_entry(struct address_space *mapping, pgoff_t offset);
> 
> No, this then leads to a panic below by LTP hugemmap05.  Still reverting the
> whole "mm: page cache: store only head pages in i_pages" commit fixed the
> problem.

Ughh... hugetlb stores pages in page cache differently.

What about this:

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index f939e004c5d1..2e8438a1216a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -335,12 +335,15 @@ static inline struct page *grab_cache_page_nowait(struct address_space *mapping,
 
 static inline struct page *find_subpage(struct page *page, pgoff_t offset)
 {
-	unsigned long index = page_index(page);
+	unsigned long mask;
+
+	if (PageHuge(page))
+		return page;
 
 	VM_BUG_ON_PAGE(PageTail(page), page);
-	VM_BUG_ON_PAGE(index > offset, page);
-	VM_BUG_ON_PAGE(index + (1 << compound_order(page)) <= offset, page);
-	return page - index + offset;
+
+	mask = (1UL << compound_order(page)) - 1;
+	return page + (offset & mask);
 }
 
 struct page *find_get_entry(struct address_space *mapping, pgoff_t offset);
-- 
 Kirill A. Shutemov