pread does not guarantee that it would read all the data asked for.
But acme usage of disk assumes that. This issues as many reads as
needed to make acme work when read returns less data than it wanted.

Reference: /n/sources/patch/applied/acme-disk-read
Date: Sat May 11 17:22:55 CES 2013
Signed-off-by: nemo@lsub.org

--- /sys/src/cmd/acme/disk.c	Sat May 11 17:21:53 2013
+++ /sys/src/cmd/acme/disk.c	Sat May 11 17:21:49 2013
@@ -120,10 +120,20 @@
 void
 diskread(Disk *d, Block *b, Rune *r, uint n)
 {
+	int tot, nr;
+	char *p;
+
 	if(n > b->n)
 		error("internal error: diskread");
 
 	ntosize(b->n, nil);
-	if(pread(d->fd, r, n*sizeof(Rune), b->addr) != n*sizeof(Rune))
+	n *= sizeof(Rune);
+	p = (char*)r;
+	for(tot = 0; tot < n; tot += nr){
+		nr = pread(d->fd, p+tot, n-tot, b->addr+tot);
+		if(nr <= 0)
+			error("read error from temp file");
+	}
+	if(tot != n)
 		error("read error from temp file");
 }