Date:	Fri, 15 Apr 2011 19:37:34 +0200
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Jan Kara <jack@...e.cz>,
	Toshiyuki Okajima <toshi.okajima@...fujitsu.com>,
	Ted Ts'o <tytso@....edu>,
	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due
 to a deadlock

On Fri 15-04-11 12:17:06, Eric Sandeen wrote:
> 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?
  I have not put that much thought to it but locking might be kind of
tricky in the generic code. We have to be sure that freezing waits for
the page which is just being faulted. That means we have to take page lock
(now writepage() called during fs_freeze will wait for us), check whether
fs is frozen. If yes, unlock page, do vfs_check_frozen(), and retry. This
call sequence is much better suited for block_page_mkwrite() than for
code in memory.c I think.

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