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From: "Li, Hao" <lihao2018.fnst@cn.fujitsu.com>
To: "linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>
Subject: Can we change the S_DAX flag immediately on XFS without dropping
 caches?
Thread-Topic: Can we change the S_DAX flag immediately on XFS without dropping
 caches?
Thread-Index: AdZkgkZYa39eKDGBSXGgU6Kg+bZxog==
Date: Tue, 28 Jul 2020 02:00:08 +0000
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CC: "Gotou, Yasunori" <y-goto@fujitsu.com>
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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.

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? I also wonder what will
happen if we set S_DAX unconditionally. Thanks!

Regards,
Hao Li


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