linux-kernel - Re: [PATCH v3 1/2] fscache,cachefiles: add prepare_ondemand

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

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

> > +static enum netfs_io_source cachefiles_do_prepare_read(struct netfs_cache_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 call,
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