RE: [RFC 08/13] cifs: Add a function to read into an iter from a socket

David Laight
2023-01-26T16:09:18Z

From: David Howells
> Sent: 26 January 2023 15:44
...
> > I'm also not 100% sure that taking a copy of an iov_iter is a good idea.
>
> It shouldn't matter as the only problematic iterator is ITER_PIPE (advancing
> that has side effects) - and splice_read is handled specially by patch 4. The
> problem with splice_read with the way cifs works is that it likes to subdivide
> its read/write requests across multiple reqs and then subsubdivide them if
> certain types of failure occur. But you can't do that with ITER_PIPE.

I was thinking that even if ok at the moment it might be troublesome later.
Somewhere I started writing a patch to put the iov_cache[] for user
requests into the same structure as the iterator.
Copying those might cause oddities.

> I build an ITER_BVEC from ITER_PIPE, ITER_UBUF and ITER_IOVEC in the top
> levels with pins inserted as appropriate and hand the ITER_BVEC down. For
> user-backed iterators it has to be done this way because the I/O may get
> shuffled off to a different thread.

For sub-page sided transfers it is probably worth doing a bounce buffer
copy of user requests - just to save all the complex page pinning code.

David

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