Re: [PATCH 3/3] RFC: dax: dax_prepare_freeze

[Boaz Harrosh <boaz@plexistor.com>]

On 03/24/2015 12:40 AM, Dave Chinner wrote:
> On Mon, Mar 23, 2015 at 02:54:40PM +0200, Boaz Harrosh wrote:
>> From: Boaz Harrosh <boaz@plexistor.com>
>>
>> When freezing an FS, we must write protect all IS_DAX()
>> inodes that have an mmap mapping on an inode. Otherwise
>> application will be able to modify previously faulted-in
>> file pages.
> 
> All you need to do is lock out page faults once the page is clean;
> that's what the sb_start_pagefault() calls are for in the page fault
> path - they catch write faults and block them until the filesystem
> is unfrozen. Hence I'm not sure why this would be necessary if you
> are catching write faults in .pfn_mkwrite....
> 

Jan pointed it out and he was right I have a test for this. What
happens is that since we had a mapping from before the freeze we will
not have a page-fault. And the buffers will be modified.

As Jan explained in the cache path we do a writeback which turns
all pages to read-only. But with dax we do not have writeback
so the pages stay read-write mapped. Something needs to loop
through the pages and write-protect them. I chose to unmap
them because it is the much-much smaller code, and I do not care
to optimize the freeze.

[Yes, sigh, I will convert the test to an xfstest. May I just add
 it to some existing fs_freeze test. Only novelty is that we need
 to write-access an mmap block before the freeze-start, then continue
 access after the freeze and see modifications
]

> Cheers,
> Dave.
> 

Thanks
Boaz