RE: [PATCH 1/2] file-posix: Correctly read max_segments of SG nodes

[Dmitry Fomichev]

> -----Original Message-----
> From: Maxim Levitsky <mlevitsk@redhat.com>
> Sent: Thursday, September 17, 2020 9:24 AM
> To: Max Reitz <mreitz@redhat.com>; Dmitry Fomichev
> <Dmitry.Fomichev@wdc.com>; Kevin Wolf <kwolf@redhat.com>; Paolo
> Bonzini <pbonzini@redhat.com>; Fam Zheng <fam@euphon.net>; Philippe
> Mathieu-Daudé <philmd@redhat.com>
> Cc: Alistair Francis <Alistair.Francis@wdc.com>; Damien Le Moal
> <Damien.LeMoal@wdc.com>; qemu-block@nongnu.org; qemu-
> devel@nongnu.org
> Subject: Re: [PATCH 1/2] file-posix: Correctly read max_segments of SG
> nodes
> 
> On Thu, 2020-09-17 at 16:22 +0300, Maxim Levitsky wrote:
> > On Thu, 2020-09-17 at 15:16 +0200, Max Reitz wrote:
> > > On 12.08.20 00:51, Dmitry Fomichev wrote:
> > > > If scsi-generic driver is in use, an SG node can be specified in
> > > > the command line in place of a regular SCSI device. In this case,
> > > > sg_get_max_segments() fails to open max_segments entry in sysfs
> > > > because /dev/sgX is a character device. As the result, the maximum
> > > > transfer size for the device may be calculated incorrectly, causing
> > > > I/O errors if the maximum transfer size at the guest ends up to be
> > > > larger compared to the host.
> > > >
> > > > Check system device type in sg_get_max_segments() and read the
> > > > max_segments value differently if it is a character device.
> > > >
> > > > Reported-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> > > > Fixes: 9103f1ceb46614b150bcbc3c9a4fbc72b47fedcc
> > > > Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
> > > > ---
> > > >  block/file-posix.c | 55 +++++++++++++++++++++++++++----------------
> ---
> > > >  1 file changed, 32 insertions(+), 23 deletions(-)
> > > >
> > > > diff --git a/block/file-posix.c b/block/file-posix.c
> > > > index 094e3b0212..f9e2424e8f 100644
> > > > --- a/block/file-posix.c
> > > > +++ b/block/file-posix.c
> > > > @@ -1108,6 +1108,7 @@ static int sg_get_max_segments(int fd)
> > > >      int ret;
> > > >      int sysfd = -1;
> > > >      long max_segments;
> > > > +    unsigned int max_segs;
> > > >      struct stat st;
> > > >
> > > >      if (fstat(fd, &st)) {
> > > > @@ -1115,30 +1116,38 @@ static int sg_get_max_segments(int fd)
> > > >          goto out;
> > > >      }
> > > >
> > > > -    sysfspath =
> g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
> > > > -                                major(st.st_rdev), minor(st.st_rdev));
> > > > -    sysfd = open(sysfspath, O_RDONLY);
> > > > -    if (sysfd == -1) {
> > > > -        ret = -errno;
> > > > -        goto out;
> > > > +    if (S_ISBLK(st.st_mode)) {
> > > > +        sysfspath =
> g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
> > > > +                                    major(st.st_rdev), 
> > > > minor(st.st_rdev));
> > >
> > > Sounds reasonable, but this function is (naturally) only called if
> > > bs->sg is true, which is set by hdev_is_sg(), which returns true only if
> > > the device file is a character device.
> > >
> > > So is this path ever taken, or can we just replace it all with the ioctl?
> > >
> > > (Before 867eccfed84, this function was used for all host devices, which
> > > might explain why the code even exists.)
> > >
> > > Max
> >
> > I have another proposal which I am currently evaluating.
> >
> > How about we drop all the SG_IO limits code alltogher from the raw driver,
> and
> > instead just let the scsi drivers (scsi-block and scsi-generic) query
> > the device directly, since I don't think that the kernel (I will double 
> > check
> this)?
> 
> I hit send too soon. I mean I don't think that the kernel imposes its own 
> limits
> on SG_IO.
> 

When I was making the fix, I had a feeling that this code should be moved
someplace else where it could be called both for block devices and sg nodes,
but that would make the patch more intrusive.

Maxim, looks like you are on top of this problem and your approach sounds
sensible too me. Just FYI, it is also possible to avoid using 
SG_GET_SG_TABLESIZE
ioctl and rely entirely on sysfs, but the code gets a bit more complicated (see 
below)

Cheers,
Dmitry

--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1110,6 +1110,8 @@ static int sg_get_max_segments(int fd)
     char buf[32];
     const char *end;
     char *sysfspath = NULL;
+    struct dirent *blkdev = NULL;
+    DIR *dir = NULL;
     int ret;
     int sysfd = -1;
     long max_segments;
@@ -1120,8 +1122,30 @@ static int sg_get_max_segments(int fd)
         goto out;
     }
 
-    sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
-                                major(st.st_rdev), minor(st.st_rdev));
+    if (S_ISBLK(st.st_mode)) {
+        sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
+                                    major(st.st_rdev), minor(st.st_rdev));
+    } else {
+        sysfspath = g_strdup_printf("/sys/dev/char/%u:%u/device/block",
+                                    major(st.st_rdev), minor(st.st_rdev));
+        dir = opendir(sysfspath);
+        if (!dir) {
+            ret = -errno;
+            goto out;
+        }
+        do {
+            blkdev = readdir(dir);
+            if (!blkdev) {
+                ret = -errno;
+                closedir(dir);
+                goto out;
+            }
+        } while (*blkdev->d_name == '.');
+        g_free(sysfspath);
+        sysfspath = g_strdup_printf("/sys/block/%s/queue/max_segments",
+                                    blkdev->d_name);
+        closedir(dir);
+    }
     sysfd = open(sysfspath, O_RDONLY);
     if (sysfd == -1) {
         ret = -errno;

> Best regards,
>       Maxim Levitsky
> >
> >
> > Best regards,
> >     Maxim Levitsky
> >
> >
>