linux-kernel - Re: [PATCH v3 23/59] netfs: Prep to use folio->private for write grouping and streaming write

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Message-ID: <ZXI6fyO1xJbLNXFg@casper.infradead.org>
Date:   Thu, 7 Dec 2023 21:34:55 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Steve French <smfrench@...il.com>,
        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 v3 23/59] netfs: Prep to use folio->private for write
 grouping and streaming write

On Thu, Dec 07, 2023 at 09:21:30PM +0000, David Howells wrote:
> +#define NETFS_FOLIO_INFO	0x1UL	/* OR'd with folio->private. */
> +
> +static inline struct netfs_folio *netfs_folio_info(struct folio *folio)
> +{
> +	void *priv = folio_get_private(folio);
> +
> +	if ((unsigned long)priv & NETFS_FOLIO_INFO)
> +		return (struct netfs_folio *)((unsigned long)priv & ~NETFS_FOLIO_INFO);

Often one gets better code by using '-' instead of '& ~', and that's
because 'subtract one, then load four bytes from offset 12' can be
optimised into 'load four bytes from offset 11' in a way that 'clear
the bottom bit, then load four bytes from offset 12' can't be.