Re: [PATCH v2] page cache: Store only head pages in i_pages

[Matthew Wilcox]

On Fri, Feb 15, 2019 at 01:29:44AM +0300, Kirill A. Shutemov wrote:
> On Thu, Feb 14, 2019 at 04:30:04PM +0300, Kirill A. Shutemov wrote:
> > On Tue, Feb 12, 2019 at 10:34:54AM -0800, Matthew Wilcox wrote:
> > > Transparent Huge Pages are currently stored in i_pages as pointers to
> > > consecutive subpages.  This patch changes that to storing consecutive
> > > pointers to the head page in preparation for storing huge pages more
> > > efficiently in i_pages.
> > > 
> > > Large parts of this are "inspired" by Kirill's patch
> > > https://lore.kernel.org/lkml/20170126115819.58875-2-kirill.shutemov@xxxxxxxxxxxxxxx/
> > > 
> > > Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > 
> > I believe I found few missing pieces:
> > 
> >  - page_cache_delete_batch() will blow up on
> > 
> > 			VM_BUG_ON_PAGE(page->index + HPAGE_PMD_NR - tail_pages
> > 					!= pvec->pages[i]->index, page);
> > 
> >  - migrate_page_move_mapping() has to be converted too.
> 
> Other missing pieces are memfd_wait_for_pins() and memfd_tag_pins()
> We need to call page_mapcount() for tail pages there.

@@ -39,6 +39,7 @@ static void memfd_tag_pins(struct xa_state *xas)
        xas_for_each(xas, page, ULONG_MAX) {
                if (xa_is_value(page))
                        continue;
+               page = find_subpage(page, xas.xa_index);
                if (page_count(page) - page_mapcount(page) > 1)
                        xas_set_mark(xas, MEMFD_TAG_PINNED);
 
@@ -88,6 +89,7 @@ static int memfd_wait_for_pins(struct address_space *mapping)
                        bool clear = true;
                        if (xa_is_value(page))
                                continue;
+                       page = find_subpage(page, xas.xa_index);
                        if (page_count(page) - page_mapcount(page) != 1) {
                                /*
                                 * On the last scan, we clean up all those tags