Subject: Writing to disk from the network stack (bottom half of the kernel)
To: None <>
From: Paul Sommerhein <>
List: tech-kern
Date: 02/04/2003 12:24:20
Ok, this might not be feasible, but what I want to do is to write
a 64kB chunk of data to the end of a disk file from within the
network stack in the bottom half of the kernel (incoming data
path, no process context).

Some assumptions

    o Not required to work with all filesystems, but need to work
      with UFS/FFS.
    o Only one writer (bottom half of kernel) and no readers (yet).
    o The file has been opened for writing by a process which
      we have a pointer to.
    o The buffer cache should not be used (for writing).
    o The 64kB chunk will (always) extend the file (appending).
    o The file system has a 64kB block size.
    o The maximum disk request is 64kB (is it?).

The idea is to allocate a buffer structure and point its data
buffer pointer to my 64kB data chunk.  Then I initialize
b_iodone to point to a function that will take care of the
buffer when the write has completed.  The other members
of the buffer structure is initalized to proper values.
The buffer is then sent to the disk driver (VOP_STRATEGY?).

If I have understood things correctly, my main problem will be
that some of the file system code might block, which I can't
tolerate since I am in the bottom half of the kernel.  More
specifically, I have a feeling that since I need to extend the
file, the process of allocating a physical disk block might block.
Is this feeling right?

Are there any other issues involved which may make this impossible?

Any pointers highly appreciated.

Best regards,
Paul Sommerhein

    I am currently taking my first stumbling steps in kernel land,
    so bear with me...