linux-ext4 - Re: [PATCH] dax: remove silly single-page limitation in dax_zero_page

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Message-ID: <CAPcyv4i5xYHFkW55eGi8L6mfoPwuMhcH3eFhDTAqzrTNvwTt4A@mail.gmail.com>
Date:   Wed, 22 Sep 2021 19:47:08 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] dax: remove silly single-page limitation in dax_zero_page_range

On Wed, Sep 22, 2021 at 6:09 PM Darrick J. Wong <djwong@...nel.org> wrote:
>
> From: Darrick J. Wong <djwong@...nel.org>
>
> It's totally silly that the dax zero_page_range implementations are
> required to accept a page count, but one of the four implementations
> silently ignores the page count and the wrapper itself errors out if you
> try to do more than one page.
>
> Fix the nvdimm implementation to loop over the page count and remove the
> artificial limitation.
>
> Signed-off-by: Darrick J. Wong <djwong@...nel.org>
> ---
>  drivers/dax/super.c   |    7 -------
>  drivers/nvdimm/pmem.c |   14 +++++++++++---
>  2 files changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index fc89e91beea7..ca61a01f9ccd 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -353,13 +353,6 @@ int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
>  {
>         if (!dax_alive(dax_dev))
>                 return -ENXIO;
> -       /*
> -        * There are no callers that want to zero more than one page as of now.
> -        * Once users are there, this check can be removed after the
> -        * device mapper code has been updated to split ranges across targets.
> -        */

It's device-mapper that's the issue, you need to make sure that every
device-mapper zero_page_range implementation knows how to route a
multi-page operation. This is part of the motivation to drop that
support and move simple concatenation and striping into the PMEM
driver directly.