Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

[Eric Sandeen <sandeen@redhat.com>]

On 4/15/11 12:13 PM, Jan Kara wrote:
>   Hello,
> 
> On Fri 15-04-11 22:39:07, Toshiyuki Okajima wrote:
>>>   For ext3 or ext4 without delayed allocation we block inside writepage()
>>> function. But as I wrote to Dave Chinner, ->page_mkwrite() should probably
>>> get modified to block while minor-faulting the page on frozen fs because
>>> when blocks are already allocated we may skip starting a transaction and so
>>> we could possibly modify the filesystem.
>> OK. I think ->page_mkwrite() should also block writing the minor-faulting pages.
>>
>> (minor-pagefault)
>> -> do_wp_page()
>>    -> page_mkwrite(= ext4_mkwrite())
>>       => BLOCK!
>>
>> (major-pagefault)
>> -> do_liner_fault()
>>    -> page_mkwrite(= ext4_mkwrite())
>>       => BLOCK!
>>
>>>
>>>>>> Mizuma-san's reproducer also writes the data which maps to the file (mmap).
>>>>>> The original problem happens after the fsfreeze operation is done.
>>>>>> I understand the normal write operation (not mmap) can be blocked while
>>>>>> fsfreezing. So, I guess we don't always block all the write operation
>>>>>> while fsfreezing.
>>>>>   Technically speaking, we block all the transaction starts which means we
>>>>> end up blocking all the writes from going to disk. But that does not mean
>>>>> we block all the writes from going to in-memory cache - as you properly
>>>>> note the mmap case is one of such exceptions.
>>>> Hm, I also think we can allow the writes to in-memory cache but we can't allow
>>>> the writes to disk while fsfreezing. I am considering that mmap path can
>>>> write to disk while fsfreezing because this deadlock problem happens after
>>>> fsfreeze operation is done...
>>>   I'm sorry I don't understand now - are you speaking about the case above
>>> when writepage() does not wait for filesystem being frozen or something
>>> else?
>> Sorry, I didn't understand around the page fault path.
>> So, I had read the kernel source code around it, then I maybe understand...
>>
>> I worry whether we can update the file data in mmap case while fsfreezing.
>> Of course, I understand that we can write to in-memory cache, and it is not a
>> problem. However, if we can write to disk while fsfreezing, it is a problem.
>> So, I summarize the cases whether we can write to disk or not.
>>
>> --------------------------------------------------------------------------
>> Cases (Whether we can write the data mmapped to the file on the disk
>> while fsfreezing)
>>
>> [1] One of the page which has been mmapped is not bound. And
>>  the page is not allocated yet. (major fault?)
>>
>>    (1) user dirtys a page
>>    (2) a page fault occurs (do_page_fault)
>>    (3) __do_falut is called.
>>    (4) ext4_page_mkwrite is called
>>    (5) ext4_write_begin is called
>>    (6) ext4_journal_start_sb       => We can STOP!
>>
>> [2] One of the page which has been mmapped is not bound. But
>>  the page is already allocated, and the buffer_heads of the page
>>  are not mapped (BH_Mapped).  (minor fault?)
>>
>>    (1) user dirtys a page
>>    (2) a page fault occurs (do_page_fault)
>>    (3) do_wp_page is called.
>>    (4) ext4_page_mkwrite is called
>>    (5) ext4_write_begin is called
>>    (6) ext4_journal_start_sb       => We can STOP!
>>
>> [3] One of the page which has been mmapped is not bound. But
>>  the page is already allocated, and the buffer_heads of the page
>>  are mapped (BH_Mapped).  (minor fault?)
>>
>>    (1) user dirtys a page
>>    (2) a page fault occurs (do_page_fault)
>>    (3) do_wp_page is called.
>>    (4) ext4_page_mkwrite is called
>>    * Cannot block the dirty page to be written because all bh is mapped.
>>    (5) user munmaps the page (munmap)
>>    (6) zap_pte_range dirtys the page (struct page) which is pte_dirtyed.
>>    (7) writeback thread writes the page (struct page) to disk
>>                                            => We cannot STOP!
>>
>> [4] One of the page which has been mmapped is bound. And
>>  the page is already allocated.
>>
>>    (1) user dirtys a page
>>    ( ) no page fault occurs
>>    (2) user munmaps the page (munmap)
>>    (3) zap_pte_range dirtys the page (struct page) which is pte_dirtyed.
>>    (4) writeback thread writes the page (struct page) to disk
>>                                            => We cannot STOP!
>> --------------------------------------------------------------------------
>>
>> So, we can block the cases [1], [2].
>> But I think we cannot block the cases [3], [4] now.
>> If fixing the page_mkwrite, we can also block the case [3].
>> But the case [4] is not blocked because no page fault occurs
>> when we dirty the mmapped page.
>>
>> Therefore, to repair this problem, we need to fix the cases [3], [4].
>> I think we must modify the writeback thread to fix the case [4].
>   The trick here is that when we write a page to disk, we write-protect
> the page (you seem to call this that "the page is bound", I'm not sure why).
> So we are guaranteed to receive a minor fault (case [3]) if user tries to
> modify a page after we finish writeback while freezing the filesystem.
> So principially all we need to do is just wait in ext4_page_mkwrite().

I've been kind of absent from this thread, sorry, but why would we wait in ext4_page_mkwrite(), rather than in mm/memory.c prior to any page_mkwrite call on any fs?

no frozen fs should be able to dirty & write pages via mmap, right?

-Eric
 
> 								Honza