linux-kernel - Re: remove a few uses of ->queuedata

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Message-ID: <20200509082352.GB21834@lst.de>
Date:   Sat, 9 May 2020 10:23:52 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Jim Paris <jim@...n.com>, Geoff Levand <geoff@...radead.org>,
        Joshua Morris <josh.h.morris@...ibm.com>,
        Philip Kelleher <pjk1939@...ux.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-m68k@...ts.linux-m68k.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xtensa@...ux-xtensa.org, drbd-dev@...ts.linbit.com,
        linux-block@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-bcache@...r.kernel.org,
        linux-raid <linux-raid@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: remove a few uses of ->queuedata

On Fri, May 08, 2020 at 11:04:45AM -0700, Dan Williams wrote:
> On Fri, May 8, 2020 at 9:16 AM Christoph Hellwig <hch@....de> wrote:
> >
> > Hi all,
> >
> > various bio based drivers use queue->queuedata despite already having
> > set up disk->private_data, which can be used just as easily.  This
> > series cleans them up to only use a single private data pointer.
> 
> ...but isn't the queue pretty much guaranteed to be cache hot and the
> gendisk cache cold? I'm not immediately seeing what else needs the
> gendisk in the I/O path. Is there another motivation I'm missing?

->private_data is right next to the ->queue pointer, pat0 and part_tbl
which are all used in the I/O submission path (generic_make_request /
generic_make_request_checks).  This is mostly a prep cleanup patch to
also remove the pointless queue argument from ->make_request - then
->queue is an extra dereference and extra churn.