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

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Message-ID: <5d0bc437eebb4d5aa4774962a4970095@AcuMS.aculab.com>
Date:   Thu, 26 Jan 2023 16:09:05 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'David Howells' <dhowells@...hat.com>
CC:     Steve French <smfrench@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Rohith Surabattula <rohiths.msft@...il.com>,
        Tom Talpey <tom@...pey.com>,
        Stefan Metzmacher <metze@...ba.org>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        "linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Steve French <sfrench@...ba.org>
Subject: RE: [RFC 08/13] cifs: Add a function to read into an iter from a
 socket

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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