On 04/18/2011 12:05 PM, Toshiyuki Okajima wrote:

Hi,

(2011/04/16 2:13), Jan Kara wrote:

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Hello,

On Fri 15-04-11 22:39:07, Toshiyuki Okajima wrote:

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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!

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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!

What happens in the case as follows:

Task 1: Mmapped writes
t1)ext4_page_mkwrite()
   t2) ext4_write_begin() (FS is thawed so we proceed)
   t3) ext4_write_end() (journal is stopped now)
-----Pre-empted-----


Task 2: Freeze Task
t4) freezes the super block...
...(continues)....
tn) the page cache is clean and the F.S is frozen. Freeze has completed 
execution.

Task 1: Mmapped writes
tn+1) ext4_page_mkwrite() returns 0.
tn+2) __do_fault() gets control, code gets executed.
tn+3) _do_fault() marks the page dirty if the intent is to write to a 
file based page which faulted.

So you end up dirtying the page cache when the F.S is frozen? No?


Warm Regards,
Surbhi.

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[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).

Hm, I want to understand how to write-protect the page under fsfreezing.
But, anyway, I understand we don't need to consider the case [4].

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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().

OK. I understand.
Are there any concrete ideas to fix this?
For ext4, we can rescue from the case [3] by modifying ext4_page_mkwrite().
But for ext3 or other FSs, we must implement ->page_mkwrite() to prevent
it?

Thanks,
Toshiyuki Okajima

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