Re: scsi disk size reporting in dmesg

[Bill Davidsen <davidsen@tmr.com>]

Randy.Dunlap wrote:
> On Thu, 20 Oct 2005, Dale Blount wrote:
>
>
> > On Wed, 2005-10-19 at 22:09 -0700, Randy.Dunlap wrote:
> >
> > > On Mon, 17 Oct 2005 15:24:04 -0400 Dale Blount wrote:
> > >
> > >
> > > > Hello,
> > > >
> > > > I just added 2 external 1TB+ scsi devices to my i686 linux server
> > > > running 2.6.13.4 connected to external LSI MPT card.  fdisk and df both
> > > > show the sizes correctly (see below), but I'm worried that dmesg reports
> > > > them incorrectly.
> > > >
> > > > SCSI device sda: 2460934144 512-byte hdwr sectors (160487 MB)
> > > > SCSI device sdb: 3790438400 512-byte hdwr sectors (841193 MB)
> > > >
> > > > I don't think it's as simple as a variable overflow because both
> > > > sdkp->capacity and mb look to be cast as unsigned long longs.  I know a
> > > > workaround is to present less data per LUN, but I'd like to use it as
> > > > it's setup currently if possible.  Is this just printing incorrectly or
> > > > will I run into trouble when the device gets more full?
> > >
> > > The casts to (unsigned long long) just fix the printk() args to match
> > > the format strings (and eliminate warnings).
> > >
> > > Looks to me like sdkp->capacity is correct.  The <mb> value looks
> > > way off.  Since it's just printed here for user info, I don't see
> > > how it can be a problem later on.
> >
> > That's what I was hoping, but I didn't know for sure.  I figured I'd
> > better ask to make sure my data wouldn't get truncated when the disk got
> > fuller.  Between my first post and now, I've tested it by filling the
> > drive with data and testing the md5sums for each file and it seems to
> > work.  Other than the odd MB size reported, it seems to work just fine.
> >
> >
> > > I don't see the error just yet.  Are there any other SCSI device-
> > > related messages near these?  And just to confirm, but you must
> > > have CONFIG_LBD (Large Block Device) enabled, right?
> >
> > No, there are no other related messages near these other than the
> > standard vendor/versions.  I did not enable CONFIG_LBD since the help
> > says "bigger than 2TB", and I partitioned the storage system to present
> > disks smaller than that.
>
>
> I guessed at CONFIG_LBD=y last night.  Today I did the arithmetic
> with CONFIG_LBD=n... and found the overflow.
>
> In drivers/scsi/sd.c, approx. line #1260:
>
> 		sector_t sz = sdkp->capacity * (hard_sector/256);
>
> sz is 32 bits.
> capacity is 32 bits with a value of 2,460,934,144 == 0x92ae_e000.
> Then we multiply that by 2 (512 / 256)... and it overflows.
> Should be hex 1_255d_c000, but we drop the '1'.
> The rest of the calculation (with this new value) does result
> in the 160487 MB, matching your log message.
>
> I think that you can fix it (patch it) by changing that line
> to make 'sz' be type 'u64' for now.
>
> 		u64 sz = sdkp->capacity * (hard_sector/256);
>
> But those divides by 1250 and 1950 are still voodoo to me.
I think that just hides the real problem, and while that's useful to 
stop the annoying numbers, the real problem is either (a) the value 
calculated is wrong, (b) the size of sector_t is no longer large enough, 
or (c) the type has been wrong all along and shouldn't be sector_t.

If I understand what sector_t really is, (b) is my candidate.

--
   -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
 last possible moment - but no longer"  -me
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