Everhart, Glenn
From:	Simon Graham [graham@mango.com]
Sent:	Tuesday, November 10, 1998 9:13 AM
To:	'Rob Green'; ntdev@atria.com
Subject:	RE: [ntdev] NT writing back to file it is reading
You need to build a Mdl for the new request out of the one in the
original Irp (using IoBuildPartialMdl); you are receiving paging reads
from above and the Mdl in the original Irp has flags set indicating this
fact (and MmUnlockPages knows not to make the pages dirty). 

If you just build your own Irp the way you are, the driver you call to
do the read will end up building a new Mdl that does NOT have these
flags set and you end up making the pages dirty in memory even for
paging reads (actually, I found when I had this bug that I would end up
with loads of pages on the Modified-No-Write list which NEVER got
cleaned).

What you should do is either pass the original Mdl through (if you do
not need to break up the original request into smaller reads to the
physical disk) or allocate a new one and use IoBuildPartialMdl() to
build Mdls that describe each portion of the original request.

Simon Graham
Mango


-----Original Message-----
From: Rob Green [mailto:robg@sundial.net]
Sent: Monday, November 09, 1998 10:55 PM
To: ntdev@atria.com
Subject: [ntdev] NT writing back to file it is reading


Hello,
   I've come across a rather odd problem.  I have a disk driver that
NTFS
calls to read data.  So far so good, i read the request data from
another
device (that isn't in control by any other driver)  , and alls well.  as
a
test i started copying a 100MB file (using copy file /b nul) after about
10
MB i start getting writes to the file that is being copied!  I have even
opened the file in read only mode and it still does it.  I have tracked
it
done to my call to read the data from the raw hard drive. (not a fdisk
format)  I use IoBuildSyncRequest to create the irp to send to the
driver.
from the pseudo code below you can see that i call
MmGetSystemAddressForMdl
then call IoBuildSync on the resulting buffer.  I think this is where
the
problem is, as IoBuildSync is going to generate another Mdl for the
buffer,
so now i have 2 mdls(the one from the Irp, and the newly created one)
and 2
systems addresses (the users buffer, and the system buffer)  Am i right
in
that this could be my problem?  I tried creating an irp using the
original
Irp, but it always BugChecks in KiInsertDeviceQueueAPC (or something
similar)  I have check the Irp APC fields and they are all null.  i used
IoAllocateIrp with a completion routine(bugcheck), and allocing my own
with
a completion routine.  The completion routine is executed, but after it
returns my main code is never restarted. (yes i return
STATUS_MORE_PROCESSING_REQUIRED)  If i check to see what file is being
read, and don't call the ReadDataFromDrive routine on it, no writes
occur
to the file.

Thanks

ReadDataFromDrive()
{
    Irp = IoBuildSynchronousRequest(...);
    if(Irp) {
        IoCallDriver(DeviceObject,Irp);
    }
}

DriverEntry(DeviceObject,Irp)
{
    Buffer = MmGetSystemAddressForMdl(Irp->MdlAddress);
    ntStatus = ReadDataFromDrive(
        DevExt,
        &ByteOffset,
        Stack->Parameters.Read.Length,
        Buffer);   
    ntStatus = DoPostProcessingOnData(
        DevExt,
        &ByteOffset,
        Stack->Parameters.Read.Length,
        Buffer);
    Irp->IoStatus.Status = ntStatus;
    Irp->IoStatus.Information = Stack->Parameters.Read.Length;
    IoCompleteRequest(Irp,IO_DISK_INCREMENT);
}
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