linux-ext4 - Re: Unexpected writes seen on a Read-only, DAX enabled EXT4 fs

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Date:   Wed, 13 Sep 2017 16:48:38 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Soccer Liu <soccer_liu@...oo.com>, linux-ext4@...r.kernel.org,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: Unexpected writes seen on a Read-only, DAX enabled EXT4 fs

On Mon 11-09-17 12:21:12, Ross Zwisler wrote:
> 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?

Well, I expect these are actually 4KB faults into private mapping (they
would end up using ext4_dax_huge_fault() as well) and the fix Ross
references below should fix that... But please tell if it does not (along
with a reproducer). Thanks!

> 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.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR