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

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Date:   Sat, 9 May 2020 08:07:14 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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 Sat, May 9, 2020 at 1:24 AM Christoph Hellwig <hch@....de> wrote:
>
> 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.

Ah ok. If the changelogs had been filled in with something like "In
preparation for removing @q from make_request_fn, stop using
->queuedata", I probably wouldn't have looked twice.

For the nvdimm/ driver updates you can add:

    Reviewed-by: Dan Williams <dan.j.williams@...el.com>

...or just let me know if you want me to pick those up through the nvdimm tree.