Re: [PATCH v2 02/25] mm: Optimise find_subpage for !THP

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Tue, Feb 11, 2020 at 11:41:05PM -0800, Christoph Hellwig wrote:
> 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).

That's for hugetlbfs:

        if (!PageCompound(page))
                return 0;

        page = compound_head(page);
        return page[1].compound_dtor == HUGETLB_PAGE_DTOR;

> 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.

Would this help?

-static inline struct page *find_subpage(struct page *page, pgoff_t offset)
+/*
+ * Given the page we found in the page cache, return the page corresponding
+ * to this offset in the file
+ */
+static inline struct page *find_subpage(struct page *head, pgoff_t offset)
 {
-       if (PageHuge(page))
-               return page;
+       /* HugeTLBfs wants the head page regardless */
+       if (PageHuge(head))
+               return head;
 
-       VM_BUG_ON_PAGE(PageTail(page), page);
+       VM_BUG_ON_PAGE(PageTail(head), head);
 
-       return page + (offset & (hpage_nr_pages(page) - 1));
+       return head + (offset & (hpage_nr_pages(head) - 1));
 }