Linux-XFS Archive on lore.kernel.org
 help / color / Atom feed
From: "Li, Hao" <lihao2018.fnst@cn.fujitsu.com>
To: Ira Weiny <ira.weiny@intel.com>
Cc: Dave Chinner <david@fromorbit.com>,
	"linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
	<yangx.jy@cn.fujitsu.com>, <ruansy.fnst@cn.fujitsu.com>,
	<gujx@cn.fujitsu.com>, Yasunori Goto <y-goto@fujitsu.com>
Subject: Re: Can we change the S_DAX flag immediately on XFS without dropping caches?
Date: Fri, 31 Jul 2020 17:12:38 +0800
Message-ID: <5717e1e5-79fb-af3c-0859-eea3cd8d9626@cn.fujitsu.com> (raw)
In-Reply-To: <20200729161040.GA1250504@iweiny-DESK2.sc.intel.com>

On 2020/7/30 0:10, Ira Weiny wrote:

> On Wed, Jul 29, 2020 at 11:23:21AM +0900, Yasunori Goto wrote:
>> Hi,
>>
>> On 2020/07/28 11:20, Dave Chinner wrote:
>>> On Tue, Jul 28, 2020 at 02:00:08AM +0000, Li, Hao wrote:
>>>> Hi,
>>>>
>>>> I have noticed that we have to drop caches to make the changing of S_DAX
>>>> flag take effect after using chattr +x to turn on DAX for a existing
>>>> regular file. The related function is xfs_diflags_to_iflags, whose
>>>> second parameter determines whether we should set S_DAX immediately.
>>> Yup, as documented in Documentation/filesystems/dax.txt. Specifically:
>>>
>>>   6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag,
>>>      the change in behaviour for existing regular files may not occur
>>>      immediately.  If the change must take effect immediately, the administrator
>>>      needs to:
>>>
>>>      a) stop the application so there are no active references to the data set
>>>         the policy change will affect
>>>
>>>      b) evict the data set from kernel caches so it will be re-instantiated when
>>>         the application is restarted. This can be achieved by:
>>>
>>>         i. drop-caches
>>>         ii. a filesystem unmount and mount cycle
>>>         iii. a system reboot
>>>
>>>> I can't figure out why we do this. Is this because the page caches in
>>>> address_space->i_pages are hard to deal with?
>>> Because of unfixable races in the page fault path that prevent
>>> changing the caching behaviour of the inode while concurrent access
>>> is possible. The only way to guarantee races can't happen is to
>>> cycle the inode out of cache.
>> I understand why the drop_cache operation is necessary. Thanks.
>>
>> BTW, even normal user becomes to able to change DAX flag for an inode,
>> drop_cache operation still requires root permission, right?
>>
>> So, if kernel have a feature for normal user can operate drop cache for "a
>> inode" with
>> its permission, I think it improve the above limitation, and
>> we would like to try to implement it recently.
>>
>> Do you have any opinion making such feature?
>> (Agree/opposition, or any other comment?)
> I would not be opposed but there were many hurdles to that implementation.
>
> What is the use case you are thinking of here?
>
> The compromise of dropping caches was reached because we envisioned that many
> users would simply want to chose the file mode when a file was created and
> maintain that mode through the lifetime of the file.  To that end one can
> simply create directories which have the desired dax mode and any files created
> in that directory will inherit the dax mode immediately.  
Inheriting mechanism for DAX mode is reasonable but chattr&drop_caches
makes things complicated.
> So there is no need
> to switch the file mode directly as a normal user.

The question is, the normal users can indeed use chattr to change the DAX
mode for a regular file as long as they want. However, when they do this,
they have no way to make the change take effect. I think this behavior is
weird. We can say chattr executes successfully because XFS_DIFLAG2_DAX has
been set onto xfs_inode->i_d.di_flags2, but we can also say chattr doesn't
finish things completely because S_DAX is not set onto inode->i_flags.
The user may be confused about why chattr +/-x doesn't work at all. Maybe
we should find a way for the normal user to make chattr take effects
without calling the administrator, or we can make the chattr +/x command
request root permission now that if the user has root permission, he can
make DAX changing take effect through echo 2 > /proc/sys/vm/drop_caches.


Regards,

Hao Li

>
> Would that work for your use case?
>
> Ira



  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-28  2:00 Li, Hao
2020-07-28  2:20 ` Dave Chinner
2020-07-29  2:23   ` Yasunori Goto
2020-07-29 16:10     ` Ira Weiny
2020-07-31  9:12       ` Li, Hao [this message]
2020-08-05  8:10         ` Li, Hao
2020-08-05 15:44           ` Darrick J. Wong
2020-08-07 16:57             ` Ira Weiny
2020-07-31 10:04       ` Yasunori Goto
2020-07-29 23:21     ` Dave Chinner
2020-07-31  9:15       ` Li, Hao
2020-07-31  9:59       ` Yasunori Goto
2020-08-07 17:09         ` Ira Weiny

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=5717e1e5-79fb-af3c-0859-eea3cd8d9626@cn.fujitsu.com \
    --to=lihao2018.fnst@cn.fujitsu.com \
    --cc=david@fromorbit.com \
    --cc=gujx@cn.fujitsu.com \
    --cc=ira.weiny@intel.com \
    --cc=linux-nvdimm@lists.01.org \
    --cc=linux-xfs@vger.kernel.org \
    --cc=ruansy.fnst@cn.fujitsu.com \
    --cc=y-goto@fujitsu.com \
    --cc=yangx.jy@cn.fujitsu.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-XFS Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-xfs/0 linux-xfs/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-xfs linux-xfs/ https://lore.kernel.org/linux-xfs \
		linux-xfs@vger.kernel.org
	public-inbox-index linux-xfs

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-xfs


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git