sg direct io/mmap oops

Back in the lk 2.4 series it was possible with the sgm_dd utility to do a zero copy transfer between two disks (or 2 locations on the same disk). That was done by doing mmap-ed IO on the read and direct IO on the write with the sg driver.

Well that no longer works in the lk 2.6 series because the sg (and st) driver uses get_user_pages() for direct IO and that returns -EFAULT when VM_IO is set on any of the user pages it is trying use. Pages that are mmap-ed have VM_IO set.

I can live with that: dio and mmap-ed IO work separately and dio can work on both sides of a transfer.

This was brought to my attention by Ahmed Teirelbar <[email protected]> with lk 2.6.5 . Worse, when tested in lk 2.6.12 and lk 2.6.13, attempting dio/mmap caused the sg driver to oops (due to empty entries in a scatter gather list). Ahmed proposed the following patch which both of us have tested.

I have alerted Kai M. as the same fix is needed in the st driver IMO.

Changelog:
- check error return (a negative integer) properly after the get_user_pages() call that sets up direct IO

Doug Gilbert

@@ -61,7 +61,7 @@
 
 #ifdef CONFIG_SCSI_PROC_FS
 #include <linux/proc_fs.h>
-static char *sg_version_date = "20050328";
+static char *sg_version_date = "20050830";
 
 static int sg_proc_init(void);
 static void sg_proc_cleanup(void);

@@ -1831,7 +1831,8 @@
 	up_read(&current->mm->mmap_sem);
 
 	/* Errors and no page mapped should return here */
-	if (res < nr_pages)
+	/* Beware of signed to unsigned integer promotions!! */
+	if ((res < 0) || (res < nr_pages))
 		goto out_unmap;
 
         for (i=0; i < nr_pages; i++) {