mm: page cache: store only head pages in i_pages

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/[email protected]/

[[email protected]: fix swapcache pages]
  Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: hugetlb stores pages in page cache differently]
  Link: http://lkml.kernel.org/r/20190404134553.vuvhgmghlkiw2hgl@kshutemo-mobl1
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox <[email protected]>
Acked-by: Jan Kara <[email protected]>
Reviewed-by: Kirill Shutemov <[email protected]>
Reviewed-and-tested-by: Song Liu <[email protected]>
Tested-by: William Kucharski <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Tested-by: Qian Cai <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Song Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index bcf909d0de5f..2e8438a1216a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -333,6 +333,19 @@ static inline struct page *grab_cache_page_nowait(struct address_space *mapping,
 			mapping_gfp_mask(mapping));
 }
 
+static inline struct page *find_subpage(struct page *page, pgoff_t offset)
+{
+	unsigned long mask;
+
+	if (PageHuge(page))
+		return page;
+
+	VM_BUG_ON_PAGE(PageTail(page), page);
+
+	mask = (1UL << compound_order(page)) - 1;
+	return page + (offset & mask);
+}
+
 struct page *find_get_entry(struct address_space *mapping, pgoff_t offset);
 struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset);
 unsigned find_get_entries(struct address_space *mapping, pgoff_t start,