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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <2b595b62f6ecd28298a860fcdc5b4941dcafd9eb.camel@kernel.org>
References: <2b595b62f6ecd28298a860fcdc5b4941dcafd9eb.camel@kernel.org> <20221116104502.107431-1-jefflexu@linux.alibaba.com> <20221116104502.107431-2-jefflexu@linux.alibaba.com>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     dhowells@redhat.com, Jingbo Xu <jefflexu@linux.alibaba.com>,
        xiang@kernel.org, chao@kernel.org, linux-erofs@lists.ozlabs.org,
        linux-cachefs@redhat.com, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v3 1/2] fscache,cachefiles: add prepare_ondemand_read() callback
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Date:   Wed, 16 Nov 2022 13:41:41 +0000
Message-ID: <2968419.1668606101@warthog.procyon.org.uk>
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Jeff Layton <jlayton@kernel.org> wrote:

> > +static enum netfs_io_source cachefiles_do_prepare_read(struct netfs_c=
ache_resources *cres,
> > +					loff_t *_start, size_t *_len,
> > +					unsigned long *_flags, loff_t i_size)
>
> _start is never changed, so it should be passed by value instead of by
> pointer.

Hmmm.  The intention was that the start pointer should be able to be moved
backwards by the cache - but that's not necessary in ->prepare_read() and
->expand_readahead() is provided for that now.  So yes, the start pointer
shouldn't get changed at this point.

> I'd also reverse the position of the arguments for _flags and i_size.
> Otherwise, the CPU/compiler have to shuffle things around more in
> cachefiles_prepare_ondemand_read before they call this.

Better to pass the flags in and then ignore them.  That way it can tail ca=
ll,
or just call cachefiles_do_prepare_read() directly from erofs.  If you're
going to have a wrapper, then you might be just as well create a
netfs_io_subrequest struct on the stack.

David