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
<ahmed.teirelbar@xxxxxxxx> 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
Signed-off-by: Douglas Gilbert <dougg@xxxxxxxxxx>
Doug Gilbert
--- linux/drivers/scsi/sg.c 2005-08-29 18:28:23.000000000 +1000
+++ linux/drivers/scsi/sg.c2613mm 2005-08-30 15:09:04.000000000 +1000
@@ -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(¤t->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++) {
