splice: use mapping_gfp_mask

The loop block driver is careful to mask __GFP_IO|__GFP_FS out of its
mapping_gfp_mask, to avoid hangs under memory pressure.  But nowadays
it uses splice, usually going through __generic_file_splice_read.  That
must use mapping_gfp_mask instead of GFP_KERNEL to avoid those hangs.
Signed-off-by: NHugh Dickins <hugh@veritas.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/splice.c

@@ -320,7 +320,7 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
 				break;
 
 			error = add_to_page_cache_lru(page, mapping, index,
-					      GFP_KERNEL);
+						mapping_gfp_mask(mapping));
 			if (unlikely(error)) {
 				page_cache_release(page);
 				if (error == -EEXIST)