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>

8 files changed, 95 insertions, 96 deletions

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index c7552459a15f..37a4d9e32cd3 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -333,6 +333,16 @@ 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)
+{
+        if (PageHuge(page))
+                return page;
+
+        VM_BUG_ON_PAGE(PageTail(page), page);
+
+        return page + (offset & (compound_nr(page) - 1));
+}
+
 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,
diff --git a/mm/filemap.c b/mm/filemap.c
index d5462d706f76..533f271d6839 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -281,11 +281,11 @@ EXPORT_SYMBOL(delete_from_page_cache);
  * @pvec: pagevec with pages to delete
  *
  * The function walks over mapping->i_pages and removes pages passed in @pvec
- * from the mapping. The function expects @pvec to be sorted by page index.
+ * from the mapping. The function expects @pvec to be sorted by page index
+ * and is optimised for it to be dense.
  * It tolerates holes in @pvec (mapping entries at those indices are not
  * modified). The function expects only THP head pages to be present in the
- * @pvec and takes care to delete all corresponding tail pages from the
- * mapping as well.
+ * @pvec.
  *
  * The function expects the i_pages lock to be held.
  */
@@ -294,40 +294,43 @@ static void page_cache_delete_batch(struct address_space *mapping,
 {
         XA_STATE(xas, &mapping->i_pages, pvec->pages[0]->index);
         int total_pages = 0;
-        int i = 0, tail_pages = 0;
+        int i = 0;
         struct page *page;
 
         mapping_set_update(&xas, mapping);
         xas_for_each(&xas, page, ULONG_MAX) {
-                if (i >= pagevec_count(pvec) && !tail_pages)
+                if (i >= pagevec_count(pvec))
                         break;
+
+                /* A swap/dax/shadow entry got inserted? Skip it. */
                 if (xa_is_value(page))
                         continue;
-                if (!tail_pages) {
-                        /*
-                         * Some page got inserted in our range? Skip it. We
-                         * have our pages locked so they are protected from
-                         * being removed.
-                         */
-                        if (page != pvec->pages[i]) {
-                                VM_BUG_ON_PAGE(page->index >
-                                                pvec->pages[i]->index, page);
-                                continue;
-                        }
-                        WARN_ON_ONCE(!PageLocked(page));
-                        if (PageTransHuge(page) && !PageHuge(page))
-                                tail_pages = HPAGE_PMD_NR - 1;
+                /*
+                 * A page got inserted in our range? Skip it. We have our
+                 * pages locked so they are protected from being removed.
+                 * If we see a page whose index is higher than ours, it
+                 * means our page has been removed, which shouldn't be
+                 * possible because we're holding the PageLock.
+                 */
+                if (page != pvec->pages[i]) {
+                        VM_BUG_ON_PAGE(page->index > pvec->pages[i]->index,
+                                        page);
+                        continue;
+                }
+
+                WARN_ON_ONCE(!PageLocked(page));
+
+                if (page->index == xas.xa_index)
                         page->mapping = NULL;
-                        /*
-                         * Leave page->index set: truncation lookup relies
-                         * upon it
-                         */
+                /* Leave page->index set: truncation lookup relies on it */
+
+                /*
+                 * Move to the next page in the vector if this is a regular
+                 * page or the index is of the last sub-page of this compound
+                 * page.
+                 */
+                if (page->index + compound_nr(page) - 1 == xas.xa_index)
                         i++;
-                } else {
-                        VM_BUG_ON_PAGE(page->index + HPAGE_PMD_NR - tail_pages
-                                        != pvec->pages[i]->index, page);
-                        tail_pages--;
-                }
                 xas_store(&xas, NULL);
                 total_pages++;
         }
@@ -1520,7 +1523,7 @@ EXPORT_SYMBOL(page_cache_prev_miss);
 struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
 {
         XA_STATE(xas, &mapping->i_pages, offset);
-        struct page *head, *page;
+        struct page *page;
 
         rcu_read_lock();
 repeat:
@@ -1535,25 +1538,19 @@ repeat:
         if (!page || xa_is_value(page))
                 goto out;
 
-        head = compound_head(page);
-        if (!page_cache_get_speculative(head))
-                goto repeat;
-
-        /* The page was split under us? */
-        if (compound_head(page) != head) {
-                put_page(head);
+        if (!page_cache_get_speculative(page))
                 goto repeat;
-        }
 
         /*
-         * Has the page moved?
+         * Has the page moved or been split?
          * This is part of the lockless pagecache protocol. See
          * include/linux/pagemap.h for details.
          */
         if (unlikely(page != xas_reload(&xas))) {
-                put_page(head);
+                put_page(page);
                 goto repeat;
         }
+        page = find_subpage(page, offset);
 out:
         rcu_read_unlock();
 
@@ -1735,7 +1732,6 @@ unsigned find_get_entries(struct address_space *mapping,
 
         rcu_read_lock();
         xas_for_each(&xas, page, ULONG_MAX) {
-                struct page *head;
                 if (xas_retry(&xas, page))
                         continue;
                 /*
@@ -1746,17 +1742,13 @@ unsigned find_get_entries(struct address_space *mapping,
                 if (xa_is_value(page))
                         goto export;
 
-                head = compound_head(page);
-                if (!page_cache_get_speculative(head))
+                if (!page_cache_get_speculative(page))
                         goto retry;
 
-                /* The page was split under us? */
-                if (compound_head(page) != head)
-                        goto put_page;
-
-                /* Has the page moved? */
+                /* Has the page moved or been split? */
                 if (unlikely(page != xas_reload(&xas)))
                         goto put_page;
+                page = find_subpage(page, xas.xa_index);
 
 export:
                 indices[ret] = xas.xa_index;
@@ -1765,7 +1757,7 @@ export:
                         break;
                 continue;
 put_page:
-                put_page(head);
+                put_page(page);
 retry:
                 xas_reset(&xas);
         }
@@ -1807,33 +1799,27 @@ unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,
 
         rcu_read_lock();
         xas_for_each(&xas, page, end) {
-                struct page *head;
                 if (xas_retry(&xas, page))
                         continue;
                 /* Skip over shadow, swap and DAX entries */
                 if (xa_is_value(page))
                         continue;
 
-                head = compound_head(page);
-                if (!page_cache_get_speculative(head))
+                if (!page_cache_get_speculative(page))
                         goto retry;
 
-                /* The page was split under us? */
-                if (compound_head(page) != head)
-                        goto put_page;
-
-                /* Has the page moved? */
+                /* Has the page moved or been split? */
                 if (unlikely(page != xas_reload(&xas)))
                         goto put_page;
 
-                pages[ret] = page;
+                pages[ret] = find_subpage(page, xas.xa_index);
                 if (++ret == nr_pages) {
                         *start = xas.xa_index + 1;
                         goto out;
                 }
                 continue;
 put_page:
-                put_page(head);
+                put_page(page);
 retry:
                 xas_reset(&xas);
         }
@@ -1878,7 +1864,6 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
 
         rcu_read_lock();
         for (page = xas_load(&xas); page; page = xas_next(&xas)) {
-                struct page *head;
                 if (xas_retry(&xas, page))
                         continue;
                 /*
@@ -1888,24 +1873,19 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
                 if (xa_is_value(page))
                         break;
 
-                head = compound_head(page);
-                if (!page_cache_get_speculative(head))
+                if (!page_cache_get_speculative(page))
                         goto retry;
 
-                /* The page was split under us? */
-                if (compound_head(page) != head)
-                        goto put_page;
-
-                /* Has the page moved? */
+                /* Has the page moved or been split? */
                 if (unlikely(page != xas_reload(&xas)))
                         goto put_page;
 
-                pages[ret] = page;
+                pages[ret] = find_subpage(page, xas.xa_index);
                 if (++ret == nr_pages)
                         break;
                 continue;
 put_page:
-                put_page(head);
+                put_page(page);
 retry:
                 xas_reset(&xas);
         }
@@ -1941,7 +1921,6 @@ unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
 
         rcu_read_lock();
         xas_for_each_marked(&xas, page, end, tag) {
-                struct page *head;
                 if (xas_retry(&xas, page))
                         continue;
                 /*
@@ -1952,26 +1931,21 @@ unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
                 if (xa_is_value(page))
                         continue;
 
-                head = compound_head(page);
-                if (!page_cache_get_speculative(head))
+                if (!page_cache_get_speculative(page))
                         goto retry;
 
-                /* The page was split under us? */
-                if (compound_head(page) != head)
-                        goto put_page;
-
-                /* Has the page moved? */
+                /* Has the page moved or been split? */
                 if (unlikely(page != xas_reload(&xas)))
                         goto put_page;
 
-                pages[ret] = page;
+                pages[ret] = find_subpage(page, xas.xa_index);
                 if (++ret == nr_pages) {
                         *index = xas.xa_index + 1;
                         goto out;
                 }
                 continue;
 put_page:
-                put_page(head);
+                put_page(page);
 retry:
                 xas_reset(&xas);
         }
@@ -2652,7 +2626,7 @@ void filemap_map_pages(struct vm_fault *vmf,
         pgoff_t last_pgoff = start_pgoff;
         unsigned long max_idx;
         XA_STATE(xas, &mapping->i_pages, start_pgoff);
-        struct page *head, *page;
+        struct page *page;
 
         rcu_read_lock();
         xas_for_each(&xas, page, end_pgoff) {
@@ -2661,24 +2635,19 @@ void filemap_map_pages(struct vm_fault *vmf,
                 if (xa_is_value(page))
                         goto next;
 
-                head = compound_head(page);
-
                 /*
                  * Check for a locked page first, as a speculative
                  * reference may adversely influence page migration.
                  */
-                if (PageLocked(head))
+                if (PageLocked(page))
                         goto next;
-                if (!page_cache_get_speculative(head))
+                if (!page_cache_get_speculative(page))
                         goto next;
 
-                /* The page was split under us? */
-                if (compound_head(page) != head)
-                        goto skip;
-
-                /* Has the page moved? */
+                /* Has the page moved or been split? */
                 if (unlikely(page != xas_reload(&xas)))
                         goto skip;
+                page = find_subpage(page, xas.xa_index);
 
                 if (!PageUptodate(page) ||
                                 PageReadahead(page) ||
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index de1f15969e27..483b07b2d6ae 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2497,6 +2497,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
         struct page *head = compound_head(page);
         pg_data_t *pgdat = page_pgdat(head);
         struct lruvec *lruvec;
+        struct address_space *swap_cache = NULL;
+        unsigned long offset = 0;
         int i;
 
         lruvec = mem_cgroup_page_lruvec(head, pgdat);
@@ -2504,6 +2506,14 @@ static void __split_huge_page(struct page *page, struct list_head *list,
         /* complete memcg works before add pages to LRU */
         mem_cgroup_split_huge_fixup(head);
 
+        if (PageAnon(head) && PageSwapCache(head)) {
+                swp_entry_t entry = { .val = page_private(head) };
+
+                offset = swp_offset(entry);
+                swap_cache = swap_address_space(entry);
+                xa_lock(&swap_cache->i_pages);
+        }
+
         for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
                 __split_huge_page_tail(head, i, lruvec, list);
                 /* Some pages can be beyond i_size: drop them from page cache */
@@ -2513,6 +2523,12 @@ static void __split_huge_page(struct page *page, struct list_head *list,
                         if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
                                 shmem_uncharge(head->mapping->host, 1);
                         put_page(head + i);
+                } else if (!PageAnon(page)) {
+                        __xa_store(&head->mapping->i_pages, head[i].index,
+                                        head + i, 0);
+                } else if (swap_cache) {
+                        __xa_store(&swap_cache->i_pages, offset + i,
+                                        head + i, 0);
                 }
         }
 
@@ -2523,10 +2539,12 @@ static void __split_huge_page(struct page *page, struct list_head *list,
         /* See comment in __split_huge_page_tail() */
         if (PageAnon(head)) {
                 /* Additional pin to swap cache */
-                if (PageSwapCache(head))
+                if (PageSwapCache(head)) {
                         page_ref_add(head, 2);
-                else
+                        xa_unlock(&swap_cache->i_pages);
+                } else {
                         page_ref_inc(head);
+                }
         } else {
                 /* Additional pin to page cache */
                 page_ref_add(head, 2);
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);
diff --git a/mm/memfd.c b/mm/memfd.c
index 650e65a46b9c..2647c898990c 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -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
diff --git a/mm/migrate.c b/mm/migrate.c
index aa72b49e0209..374ef2fcb722 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -460,7 +460,7 @@ int migrate_page_move_mapping(struct address_space *mapping,
 
                 for (i = 1; i < HPAGE_PMD_NR; i++) {
                         xas_next(&xas);
-                        xas_store(&xas, newpage + i);
+                        xas_store(&xas, newpage);
                 }
         }
 
diff --git a/mm/shmem.c b/mm/shmem.c
index 15d26c86e5ef..57a6aedf6649 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -631,7 +631,7 @@ static int shmem_add_to_page_cache(struct page *page,
                 if (xas_error(&xas))
                         goto unlock;
 next:
-                xas_store(&xas, page + i);
+                xas_store(&xas, page);
                 if (++i < nr) {
                         xas_next(&xas);
                         goto next;
diff --git a/mm/swap_state.c b/mm/swap_state.c
index f844af5f09ba..8e7ce9a9bc5e 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -133,7 +133,7 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp)
                 for (i = 0; i < nr; i++) {
                         VM_BUG_ON_PAGE(xas.xa_index != idx + i, page);
                         set_page_private(page + i, entry.val + i);
-                        xas_store(&xas, page + i);
+                        xas_store(&xas, page);
                         xas_next(&xas);
                 }
                 address_space->nrpages += nr;
@@ -168,7 +168,7 @@ void __delete_from_swap_cache(struct page *page, swp_entry_t entry)
 
         for (i = 0; i < nr; i++) {
                 void *entry = xas_store(&xas, NULL);
-                VM_BUG_ON_PAGE(entry != page + i, entry);
+                VM_BUG_ON_PAGE(entry != page, entry);
                 set_page_private(page + i, 0);
                 xas_next(&xas);
         }