netdev - Re: [PATCH v4 28/39] netfs: Implement support for unbuffered/DIO read

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Date: Tue, 19 Dec 2023 15:46:28 +0000
From: David Howells <dhowells@...hat.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: dhowells@...hat.com, Steve French <smfrench@...il.com>,
    Matthew Wilcox <willy@...radead.org>,
    Marc Dionne <marc.dionne@...istor.com>,
    Paulo Alcantara <pc@...guebit.com>,
    Shyam Prasad N <sprasad@...rosoft.com>, Tom Talpey <tom@...pey.com>,
    Dominique Martinet <asmadeus@...ewreck.org>,
    Eric Van Hensbergen <ericvh@...nel.org>,
    Ilya Dryomov <idryomov@...il.com>,
    Christian Brauner <christian@...uner.io>, linux-cachefs@...hat.com,
    linux-afs@...ts.infradead.org, linux-cifs@...r.kernel.org,
    linux-nfs@...r.kernel.org, ceph-devel@...r.kernel.org,
    v9fs@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
    linux-mm@...ck.org, netdev@...r.kernel.org,
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 28/39] netfs: Implement support for unbuffered/DIO read

Jeff Layton <jlayton@...nel.org> wrote:

> > +static int netfs_copy_xarray_to_iter(struct netfs_io_request *rreq,
> > +				     struct xarray *xa, struct iov_iter *dst,
> > +				     unsigned long long start, size_t avail)
> ...
> > +	xas_for_each(&xas, folio, ULONG_MAX) {
> > +		size_t offset;
> > +
> > +		if (xas_retry(&xas, folio))
> > +			continue;
> > +
> > +		/* There shouldn't be a need to call xas_pause() as no one else
> > +		 * should be modifying the xarray we're iterating over.
> > +		 * Really, we only need the RCU readlock to keep lockdep happy
> > +		 * inside xas_for_each().
> > +		 */
> > +		rcu_read_unlock();
> > +
> 
> Are you sure it's still safe to access "folio" once you've dropped the
> rcu_read_lock? I wonder if you need to take a reference or something.
> 
> I guess if this is a "private" xarray then nothing should be modifying
> it?

It is a private xarray in this case.  The only reason I need the RCU read lock
is because the xarray code will complain if we're not holding it:-/.

Actually, I can probably excise the bounce buffer code for this series as
nothing in 9p or afs currently uses it.  It's needed for content crypto - so
ceph will need it.

David