From: Ross Zwisler Subject: Re: Unexpected writes seen on a Read-only, DAX enabled EXT4 fs Date: Mon, 11 Sep 2017 12:21:12 -0600 Message-ID: <20170911182112.GA20005@linux.intel.com> References: <2036030243.251059.1504977026265.ref@mail.yahoo.com> <2036030243.251059.1504977026265@mail.yahoo.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Cc: "linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org" To: Soccer Liu , linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Return-path: Content-Disposition: inline In-Reply-To: <2036030243.251059.1504977026265-sAHhhX/85wgbqTNvkayDYw@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org On Sat, Sep 09, 2017 at 05:10:26PM +0000, Soccer Liu wrote: > Hi: > =A0 I was surprised to see a few write operations called into > ext4_dax_huge_fault with FAULT_FLAG_WRITE set on vmf->flags > Are those writes expected at all? > static int ext4_dax_huge_fault(struct vm_fault *vmf, > enum page_entry_size pe_size) > { > ... > bool write =3D vmf->flags & FAULT_FLAG_WRITE; > Based on the following Mount documentation below, I did mount this ext fs > with ro and noloadAre there any other options that I could use to totally > eliminate those unexpected writes? > = > - -r, --read-only > - Mount the filesystem read-only. A synonym is -o ro.Note that, > depending on the filesystem type, state and kernel behavior, the > system may still write to the device. For example, Ext3 or ext4 will > replay itsjournal if the filesystem is dirty. To prevent this kind = of > write access, you may want to mount ext3 or ext4 filesystem with > "ro,noload" mount options or setthe block device to read-only mode, > see command blockdev(8). - = > = > Related DMESG output... > = > [=A0=A0=A0 0.891653] EXT4-fs (pmem0): DAX enabled. Warning: EXPERIMENTAL,= use at your own risk > [=A0=A0=A0 0.892736] EXT4-fs (pmem0): mounted filesystem without journal.= Opts: noload,dax > [=A0=A0=A0 0.898293] EXT4-fs (sda): mounted filesystem without journal. O= pts: (null) > [=A0=A0=A0 0.902574] random: fast init done > [=A0=A0=A0 0.926157] IPVS: ftp: loaded support on port[0] =3D 21 > // My private debug messages > [=A0=A0=A0 0.997224] EXT4-fs (pmem0): write is 1 handle =3D 0xfffffffffff= fffe2 <--read-only device=A0 > [=A0=A0=A0 0.998052] EXT4-fs (pmem0): write is 1 handle =3D 0xfffffffffff= fffe2 > [=A0=A0=A0 0.998787] EXT4-fs (pmem0): write is 1 handle =3D 0xfffffffffff= fffe2 > [=A0=A0=A0 0.999720] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0=A0 1.004626] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0=A0 1.005359] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0=A0 1.006128] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0=A0 1.006824] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0=A0 1.007499] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0=A0 1.008149] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0=A0 7.320626] random: crng init done > [=A0=A0 18.780447] EXT4-fs: 53 callbacks suppressed > [=A0=A0 18.781900] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0 18.783392] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0 18.790661] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0 18.792042] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0 18.793321] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0 18.795005] EXT4-fs (pmem0): write is 0 handle =3D 0x0 > [=A0=A0 18.795204] EXT4-fs (pmem0): write is 1 handle =3D 0xfffffffffffff= fe2 > [=A0=A0 18.795209] EXT4-fs (pmem0): write is 1 handle =3D 0xfffffffffffff= fe2 > [=A0=A0 18.795213] EXT4-fs (pmem0): write is 1 handle =3D 0xfffffffffffff= fe2 > [=A0=A0 18.795257] EXT4-fs (pmem0): write is 0 handle =3D 0x0 Adding the ext4 list. I'm pretty sure that this is ext4/mm behavior that is independent of DAX. = DAX doesn't really have any impact on whether or not write page faults happen o= n a RO ext4 filesystem, though this fix from Randy Dodgen: https://www.spinics.net/lists/linux-ext4/msg58075.html shows that FAULT_FLAG_WRITE can be set on DAX read-only ext4 mounts when executing binaries, though in this case the write is going to a COW page. - Ross