erofs: use read_cache_page_gfp for erofs_get_meta_page

As Christoph said [1], "I'd much prefer to just use
read_cache_page_gfp, and live with the fact that this
allocates bufferheads behind you for now.  I'll try to
speed up my attempts to get rid of the buffer heads on
the block device mapping instead. "

This simplifies the code a lot and a minor thing is
"no REQ_META (e.g. for blktrace) on metadata at all..."

[1] https://lore.kernel.org/r/20190903153704.GA2201@infradead.org/
Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-26-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/erofs/data.c

@@ -32,71 +32,13 @@ static void erofs_readendio(struct bio *bio)
 	bio_put(bio);
 }
 
-static struct bio *erofs_grab_raw_bio(struct super_block *sb,
-				      erofs_blk_t blkaddr,
-				      unsigned int nr_pages,
-				      bool ismeta)
-{
-	struct bio *bio = bio_alloc(GFP_NOIO, nr_pages);
-
-	bio->bi_end_io = erofs_readendio;
-	bio_set_dev(bio, sb->s_bdev);
-	bio->bi_iter.bi_sector = (sector_t)blkaddr << LOG_SECTORS_PER_BLOCK;
-	if (ismeta)
-		bio->bi_opf = REQ_OP_READ | REQ_META;
-	else
-		bio->bi_opf = REQ_OP_READ;
-
-	return bio;
-}
-
 struct page *erofs_get_meta_page(struct super_block *sb, erofs_blk_t blkaddr)
 {
 	struct inode *const bd_inode = sb->s_bdev->bd_inode;
 	struct address_space *const mapping = bd_inode->i_mapping;
-	const gfp_t gfp = mapping_gfp_constraint(mapping, ~__GFP_FS);
-	struct page *page;
-	int err;
-
-repeat:
-	page = find_or_create_page(mapping, blkaddr, gfp);
-	if (!page)
-		return ERR_PTR(-ENOMEM);
-
-	DBG_BUGON(!PageLocked(page));
-
-	if (!PageUptodate(page)) {
-		struct bio *bio;
-
-		bio = erofs_grab_raw_bio(sb, blkaddr, 1, true);
-
-		if (bio_add_page(bio, page, PAGE_SIZE, 0) != PAGE_SIZE) {
-			err = -EFAULT;
-			goto err_out;
-		}
-
-		submit_bio(bio);
-		lock_page(page);
 
-		/* this page has been truncated by others */
-		if (page->mapping != mapping) {
-			unlock_page(page);
-			put_page(page);
-			goto repeat;
-		}
-
-		/* more likely a read error */
-		if (!PageUptodate(page)) {
-			err = -EIO;
-			goto err_out;
-		}
-	}
-	return page;
-
-err_out:
-	unlock_page(page);
-	put_page(page);
-	return ERR_PTR(err);
+	return read_cache_page_gfp(mapping, blkaddr,
+				   mapping_gfp_constraint(mapping, ~__GFP_FS));
 }
 
 static int erofs_map_blocks_flatmode(struct inode *inode,
@@ -272,7 +214,13 @@ static inline struct bio *erofs_read_raw_page(struct bio *bio,
 		if (nblocks > BIO_MAX_PAGES)
 			nblocks = BIO_MAX_PAGES;
 
-		bio = erofs_grab_raw_bio(sb, blknr, nblocks, false);
+		bio = bio_alloc(GFP_NOIO, nblocks);
+
+		bio->bi_end_io = erofs_readendio;
+		bio_set_dev(bio, sb->s_bdev);
+		bio->bi_iter.bi_sector = (sector_t)blknr <<
+			LOG_SECTORS_PER_BLOCK;
+		bio->bi_opf = REQ_OP_READ;
 	}
 
 	err = bio_add_page(bio, page, PAGE_SIZE, 0);