Re: Unexpected writes seen on a Read-only, DAX enabled EXT4 fs

[Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Sat, Sep 09, 2017 at 05:10:26PM +0000, Soccer Liu wrote:
> Hi:
>   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 = 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...
> 
> [    0.891653] EXT4-fs (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
> [    0.892736] EXT4-fs (pmem0): mounted filesystem without journal. Opts: noload,dax
> [    0.898293] EXT4-fs (sda): mounted filesystem without journal. Opts: (null)
> [    0.902574] random: fast init done
> [    0.926157] IPVS: ftp: loaded support on port[0] = 21
> // My private debug messages
> [    0.997224] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2 <--read-only device 
> [    0.998052] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
> [    0.998787] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
> [    0.999720] EXT4-fs (pmem0): write is 0 handle = 0x0
> [    1.004626] EXT4-fs (pmem0): write is 0 handle = 0x0
> [    1.005359] EXT4-fs (pmem0): write is 0 handle = 0x0
> [    1.006128] EXT4-fs (pmem0): write is 0 handle = 0x0
> [    1.006824] EXT4-fs (pmem0): write is 0 handle = 0x0
> [    1.007499] EXT4-fs (pmem0): write is 0 handle = 0x0
> [    1.008149] EXT4-fs (pmem0): write is 0 handle = 0x0
> [    7.320626] random: crng init done
> [   18.780447] EXT4-fs: 53 callbacks suppressed
> [   18.781900] EXT4-fs (pmem0): write is 0 handle = 0x0
> [   18.783392] EXT4-fs (pmem0): write is 0 handle = 0x0
> [   18.790661] EXT4-fs (pmem0): write is 0 handle = 0x0
> [   18.792042] EXT4-fs (pmem0): write is 0 handle = 0x0
> [   18.793321] EXT4-fs (pmem0): write is 0 handle = 0x0
> [   18.795005] EXT4-fs (pmem0): write is 0 handle = 0x0
> [   18.795204] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
> [   18.795209] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
> [   18.795213] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
> [   18.795257] EXT4-fs (pmem0): write is 0 handle = 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 on 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