Re: [PATCH 3/3 v2] dax: use pfn_mkwrite to update c/mtime + freeze protection

[Boaz Harrosh <boaz@plexistor.com>]

On 03/05/2015 12:35 PM, Jan Kara wrote:
> On Thu 05-03-15 11:32:25, Boaz Harrosh wrote:
>> On 03/05/2015 11:24 AM, Boaz Harrosh wrote:
<>
>>
>> Just as curiosity, does the freezing code goes and turns all mappings
>> into read-only, Also for pfn mapping?
>   Hum, that's a good question. Probably we don't end up doing that. For
>
> normal filesystems we sync all inodes which also writeprotects all pages
> (in clear_page_dirty_for_io() - for normal filesystems we know that if page
> is writeably mapped it is dirty). However this won't happen for pfn
> mapping as we don't have dirty pages. So we probably need dax_freeze()
> implementation that will walk through all inodes with writeable mappings and
> writeprotect them.
> 

I'll go head and try my shot on implementing a dax_freeze(). But I will
please need help with where to call it from.

Probably something like:
	if (IS_DAX(inode))
		dax_freeze(inode);
	else
		sync(inode)

So to share the for-all-inodes-in-sb loop. And also the IS_DAX(inode)
is per inode, for example dirs need the regular sync (if they are using page cache)

>> Do you think there is already an xfstest freezing test that should now
>> fail, and will succeed after this patch (v2). Something like:
>>   * mmap-read/write before the freeze
>>   * freeze the fs
>>   * Another thread tries to mmap-write, should get stuck
>>   * unfreeze the fs
>>   * Now mmap-writer continues
>   I don't remember there would be any test to specifically test this.
> 

OK Thanks, I was hopping we should already test for mmap vs freeze. This
is not special for our case. Actually mmap is the most fragile access.

> 								Honza
> 

Thanks
Boaz

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