[patch rfc] towards supporting O_NONBLOCK on regular files

[Jeff Moyer]

This patch makes an attempt at supporting the O_NONBLOCK flag for regular
files.  It's pretty straight-forward.  One limitation is that we still call
into the readahead code, which I believe can block.  However, if we don't
do this, then an application which only uses non-blocking reads may never
get it's data.

Comments welcome.

-Jeff

--- linux-2.6.8/mm/filemap.c.orig	2004-09-30 16:33:46.881129560 -0400
+++ linux-2.6.8/mm/filemap.c	2004-09-30 16:34:12.109294296 -0400
@@ -720,7 +720,7 @@ void do_generic_mapping_read(struct addr
 	unsigned long index, end_index, offset;
 	loff_t isize;
 	struct page *cached_page;
-	int error;
+	int error, nonblock = filp->f_flags & O_NONBLOCK;
 	struct file_ra_state ra = *_ra;
 
 	cached_page = NULL;
@@ -755,10 +755,20 @@ find_page:
 		page = find_get_page(mapping, index);
 		if (unlikely(page == NULL)) {
 			handle_ra_miss(mapping, &ra, index);
+			if (nonblock) {
+				desc->error = -EWOULDBLOCK;
+				break;
+			}
 			goto no_cached_page;
 		}
-		if (!PageUptodate(page))
+		if (!PageUptodate(page)) {
+			if (nonblock) {
+				page_cache_release(page);
+				desc->error = -EWOULDBLOCK;
+				break;
+			}
 			goto page_not_up_to_date;
+		}
 page_ok:
 
 		/* If users can be writing to this page using arbitrary
@@ -1004,7 +1014,7 @@ __generic_file_aio_read(struct kiocb *io
 			desc.error = 0;
 			do_generic_file_read(filp,ppos,&desc,file_read_actor);
 			retval += desc.written;
-			if (!retval) {
+			if (!retval || desc.error) {
 				retval = desc.error;
 				break;
 			}
-
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