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From: David Howells <dhowells@redhat.com>
In-Reply-To: <36ba1d9f8668e701a9eebcc6cbaa9367e7ccb182.camel@kernel.org>
References: <36ba1d9f8668e701a9eebcc6cbaa9367e7ccb182.camel@kernel.org> <20231213152350.431591-1-dhowells@redhat.com> <20231213152350.431591-29-dhowells@redhat.com>
To: Jeff Layton <jlayton@kernel.org>
Cc: dhowells@redhat.com, Steve French <smfrench@gmail.com>,
    Matthew Wilcox <willy@infradead.org>,
    Marc Dionne <marc.dionne@auristor.com>,
    Paulo Alcantara <pc@manguebit.com>,
    Shyam Prasad N <sprasad@microsoft.com>, Tom Talpey <tom@talpey.com>,
    Dominique Martinet <asmadeus@codewreck.org>,
    Eric Van Hensbergen <ericvh@kernel.org>,
    Ilya Dryomov <idryomov@gmail.com>,
    Christian Brauner <christian@brauner.io>, linux-cachefs@redhat.com,
    linux-afs@lists.infradead.org, linux-cifs@vger.kernel.org,
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Subject: Re: [PATCH v4 28/39] netfs: Implement support for unbuffered/DIO read
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Content-ID: <844304.1703000788.1@warthog.procyon.org.uk>
Date: Tue, 19 Dec 2023 15:46:28 +0000
Message-ID: <844305.1703000788@warthog.procyon.org.uk>

Jeff Layton <jlayton@kernel.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