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

[Jan Kara <jack@suse.cz>]

  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().

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR