Matthew Dillon wrote...
> :I have the following question: Let's say that I have a block of user
> :memory which I've mapped into the kernel, and would like to send on a
> :network socket. I'd like to simply grab an mbuf, point to the memory as
> :external storage, and queue it up for transmission. This would work fine,
> :except that when MFREE gets called, I have to write an deallocator that
> :maintains a table of all the different cases where I've done this, and do
> :a reverse mapping back to the original block, and then deal with sending
> :more, unmapping, etc. In other words, having MFREE call a deallocator
> :with just the data pointer and the size is inconvenient (actually, it
> :would make my scenario quite inefficient given the number of mappings back
> :to the original block that would have to be done).
> :
> :Am I missing another mechanism to handle this? Does it not come up enough
> :to matter?
> :
> :-Chris
>
> This is almost precisely the mechanism that the sendfile() system call
> uses. In that case it maps VMIO-backed data rather then user memory,
> but it is a very similar problem.
>
> There has been talk of implementing this type of mechanism not only for
> sockets, but for file read()/write() as well. In fact, John Dyson had
> delved into the issue with his vfs.ioopt stuff before he ran out of time.
>
> The one problem with using direct VM page mappings is that currently there
> is no way for the socket to prevent the underlying data from being
> modified in the middle of a transmission. And, in the same respect for
> vfs.ioopt, no way to prevent the data the user ostensibly read() into
> his 'private' buffer from changing out from under the user if the
> underlying file is modified.
How about marking the page copy-on-write? That way, if the user modifies
the page while it is being transmitted, it'll just be copied, so the
original data will be intact.
Ken
--
Kenneth Merry
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