f2fs: add customized migrate_page callback

This patch improves the migration of dirty pages and allows migrating atomic
written pages that F2FS uses in Page Cache. Instead of the fallback releasing
page path, it provides better performance for memory compaction, CMA and other
users of memory page migrating. For dirty pages, there is no need to write back
first when migrating. For an atomic written page before committing, we can
migrate the page and update the related 'inmem_pages' list at the same time.

Signed-off-by: Weichao Guo <guoweichao@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: fix some coding style]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1883,6 +1883,58 @@ static sector_t f2fs_bmap(struct address_space *mapping, sector_t block)
 	return generic_block_bmap(mapping, block, get_data_block_bmap);
 }
 
+#ifdef CONFIG_MIGRATION
+#include <linux/migrate.h>
+
+int f2fs_migrate_page(struct address_space *mapping,
+		struct page *newpage, struct page *page, enum migrate_mode mode)
+{
+	int rc, extra_count;
+	struct f2fs_inode_info *fi = F2FS_I(mapping->host);
+	bool atomic_written = IS_ATOMIC_WRITTEN_PAGE(page);
+
+	BUG_ON(PageWriteback(page));
+
+	/* migrating an atomic written page is safe with the inmem_lock hold */
+	if (atomic_written && !mutex_trylock(&fi->inmem_lock))
+		return -EAGAIN;
+
+	/*
+	 * A reference is expected if PagePrivate set when move mapping,
+	 * however F2FS breaks this for maintaining dirty page counts when
+	 * truncating pages. So here adjusting the 'extra_count' make it work.
+	 */
+	extra_count = (atomic_written ? 1 : 0) - page_has_private(page);
+	rc = migrate_page_move_mapping(mapping, newpage,
+				page, NULL, mode, extra_count);
+	if (rc != MIGRATEPAGE_SUCCESS) {
+		if (atomic_written)
+			mutex_unlock(&fi->inmem_lock);
+		return rc;
+	}
+
+	if (atomic_written) {
+		struct inmem_pages *cur;
+		list_for_each_entry(cur, &fi->inmem_pages, list)
+			if (cur->page == page) {
+				cur->page = newpage;
+				break;
+			}
+		mutex_unlock(&fi->inmem_lock);
+		put_page(page);
+		get_page(newpage);
+	}
+
+	if (PagePrivate(page))
+		SetPagePrivate(newpage);
+	set_page_private(newpage, page_private(page));
+
+	migrate_page_copy(newpage, page);
+
+	return MIGRATEPAGE_SUCCESS;
+}
+#endif
+
 const struct address_space_operations f2fs_dblock_aops = {
 	.readpage	= f2fs_read_data_page,
 	.readpages	= f2fs_read_data_pages,
@@ -1895,4 +1947,7 @@ const struct address_space_operations f2fs_dblock_aops = {
 	.releasepage	= f2fs_release_page,
 	.direct_IO	= f2fs_direct_IO,
 	.bmap		= f2fs_bmap,
+#ifdef CONFIG_MIGRATION
+	.migratepage    = f2fs_migrate_page,
+#endif
 };