iomap: Handle memory allocation failure in readahead

bio_alloc() can fail when we use GFP_NORETRY.  If it does, allocate
a bio large enough for a single page like mpage_readpages() does.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/iomap/buffered-io.c

@@ -302,6 +302,7 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
 
 	if (!ctx->bio || !is_contig || bio_full(ctx->bio, plen)) {
 		gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
+		gfp_t orig_gfp = gfp;
 		int nr_vecs = (length + PAGE_SIZE - 1) >> PAGE_SHIFT;
 
 		if (ctx->bio)
@@ -310,6 +311,13 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
 		if (ctx->is_readahead) /* same as readahead_gfp_mask */
 			gfp |= __GFP_NORETRY | __GFP_NOWARN;
 		ctx->bio = bio_alloc(gfp, min(BIO_MAX_PAGES, nr_vecs));
+		/*
+		 * If the bio_alloc fails, try it again for a single page to
+		 * avoid having to deal with partial page reads.  This emulates
+		 * what do_mpage_readpage does.
+		 */
+		if (!ctx->bio)
+			ctx->bio = bio_alloc(orig_gfp, 1);
 		ctx->bio->bi_opf = REQ_OP_READ;
 		if (ctx->is_readahead)
 			ctx->bio->bi_opf |= REQ_RAHEAD;