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/20170126115819.58875-2-kirill.shutemov@linux.intel.com/

Kirill and Huang Ying contributed several fixes.

[willy@infradead.org: use compound_nr, squish uninit-var warning]
Link: http://lkml.kernel.org/r/20190731210400.7419-1-willy@infradead.org
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Jan Kara <jack@suse.cz>
Reviewed-by: Kirill Shutemov <kirill@shutemov.name>
Reviewed-by: Song Liu <songliubraving@fb.com>
Tested-by: Song Liu <songliubraving@fb.com>
Tested-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Tested-by: Qian Cai <cai@lca.pw>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Song Liu <songliubraving@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ccede2425c3f..04a54ff5a8ac 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1378,7 +1378,7 @@ static void collapse_shmem(struct mm_struct *mm,
                                 result = SCAN_FAIL;
                                 goto xa_locked;
                         }
-                        xas_store(&xas, new_page + (index % HPAGE_PMD_NR));
+                        xas_store(&xas, new_page);
                         nr_none++;
                         continue;
                 }
@@ -1454,7 +1454,7 @@ static void collapse_shmem(struct mm_struct *mm,
                 list_add_tail(&page->lru, &pagelist);
 
                 /* Finally, replace with the new page. */
-                xas_store(&xas, new_page + (index % HPAGE_PMD_NR));
+                xas_store(&xas, new_page);
                 continue;
 out_unlock:
                 unlock_page(page);