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Message-ID: <CAPcyv4h0QdHi10ngaXtuisxeZ+66wd-oy0F7r9C0FjJmyXBOFg@mail.gmail.com>
Date: Mon, 23 Aug 2021 11:45:52 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Christoph Hellwig <hch@....de>
Cc: 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 1/9] fsdax: improve the FS_DAX Kconfig description and
help text
On Mon, Aug 23, 2021 at 5:37 AM Christoph Hellwig <hch@....de> wrote:
>
> Rename the main option text to clarify it is for file system access,
> and add a bit of text that explains how to actually switch a nvdimm
> to a fsdax capable state.
>
Looks good, nice improvement. A couple suggestions below.
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
> fs/Kconfig | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/fs/Kconfig b/fs/Kconfig
> index a7749c126b8e..37e4441119cf 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -43,7 +43,7 @@ source "fs/f2fs/Kconfig"
> source "fs/zonefs/Kconfig"
>
> config FS_DAX
> - bool "Direct Access (DAX) support"
> + bool "File system based Direct Access (DAX) support"
> depends on MMU
> depends on !(ARM || MIPS || SPARC)
> select DEV_PAGEMAP_OPS if (ZONE_DEVICE && !FS_DAX_LIMITED)
> @@ -53,8 +53,19 @@ config FS_DAX
> Direct Access (DAX) can be used on memory-backed block devices.
> If the block device supports DAX and the filesystem supports DAX,
> then you can avoid using the pagecache to buffer I/Os. Turning
> - on this option will compile in support for DAX; you will need to
> - mount the filesystem using the -o dax option.
> + on this option will compile in support for DAX.
> +
> + For a DAX device to support file system access it needs to have
> + struct pages. For the nfit based NVDIMMs this can be enabled
> + using the ndctl utility:
> +
> + # ndctl create-namespace --force --reconfig=namespace0.0 \
> + --mode=fsdax --map=mem
There's still the concern that on systems with small amount of DRAM
relative to large amounts of PMEM that --map=mem might consume all
available memory for 'struct page'. Perhaps just add:
"See the 'create-namespace' man page for details on the overhead of
--map=mem: https://docs.pmem.io/ndctl-user-guide/ndctl-man-pages/ndctl-create-namespace"
> +
> + For ndctl to work CONFIG_DEV_DAX needs to be enabled as well.
> + For most file systems DAX support needs to be manually enable
> + globally or per-inode using a mount option as well. See the
> + file system documentation for details.
How about include the link?
"See the file system documentation for details:
https://www.kernel.org/doc/html/latest/filesystems/dax.html"
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