linux-ext4 - Re: your mail

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Date:	Tue, 03 May 2011 16:56:57 +0300
From:	Surbhi Palande <surbhi.palande@...onical.com>
To:	Jan Kara <jack@...e.cz>
CC:	toshi.okajima@...fujitsu.com, tytso@....edu,
	m.mizuma@...fujitsu.com, adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: your mail

On 05/03/2011 04:46 PM, Jan Kara wrote:
> On Tue 03-05-11 16:08:36, Surbhi Palande wrote:

Sorry for missing the subject line :(
>> On munmap() zap_pte_range() is called which dirties the PTE dirty pages as
>> Toshiyuki pointed out.
>>
>> zap_pte_range()
>>    mapping->a_ops->set_page_dirty (= ext4_journalled_set_page_dirty)
>>
>> So, I think that it is here that we should do the checking for a ext4 F.S
>> frozen state and also prevent a parallel ext4 F.S freeze from happening.
>>
>> Attaching a patch for initial review. Please do let me know your thoughts!
>    This is definitely the wrong place. ->set_page_dirty() callbacks are
> called with various locks held and the page need not be locked (thus
> dereferencing page->mapping is oopsable). Moreover this particular callback
> is called only in data=journal mode.
Ok! Thanks for that!

>
> Believe me, the right place is page_mkwrite() - you have to catch the
> read-only =>  read-write page transition. Once the page is mapped
> read-write, you've already lost the race.

My only point is:
1) something should prevent the freeze from happening. We cant merely 
check the vfs_check_frozen()?

And this should be done where the page is marked dirty.Also, I thought 
that the page is marked read-write only in the page table in the 
__do_page_fault()? i.e the zap_pte_range() marks them dirty in the page 
cache? Is this understanding right?

IMHO, whatever code dirties the page in the page cache should call a F.S 
specific function and let it _prevent_ a fsfreeze while the page is 
getting dirtied, so that a freeze called after this point flushes this page!

Warm Regards,
Surbhi.










>
> 								Honza

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