linux-ext4 - [Bug 15827] New: ext4_get_blocks may be called while ext4

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Message-ID: <bug-15827-13602@https.bugzilla.kernel.org/>
Date:	Thu, 22 Apr 2010 04:36:14 GMT
From:	bugzilla-daemon@...zilla.kernel.org
To:	linux-ext4@...r.kernel.org
Subject: [Bug 15827] New: ext4_get_blocks may be called while 
 ext4_truncate() is in progress

https://bugzilla.kernel.org/show_bug.cgi?id=15827

           Summary: ext4_get_blocks may be called while  ext4_truncate()
                    is in progress
           Product: File System
           Version: 2.5
    Kernel Version: v2.6.29-rc5
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: ext4
        AssignedTo: fs_ext4@...nel-bugs.osdl.org
        ReportedBy: dmonakhov@...nvz.org
                CC: jack@...e.cz, dmonakhov@...nvz.org
        Regression: No



Dmitry Monakhov <dmonakhov@...nvz.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dmonakhov@...nvz.org
                 CC|                            |jack@...e.cz


Created an attachment (id=26081)
 --> (https://bugzilla.kernel.org/attachment.cgi?id=26081)
dmesg output

During truncate we may need to restart new transaction, to avoid deadlock on
i_data_sem it was dropped 
commit 487caeef9fc08c0565e082c40a8aaf58dad92bbb
Author: Jan Kara <jack@...e.cz>
Date:   Mon Aug 17 22:17:20 2009 -0400

Jan given a good explanation why this approach would work, I have better
explanation why this can't work work.
Yes we have blocked all writers beyond i_size, but writers(flush, page_mkwrite)
before i_size still may change node blocks, so 'path' which was lookup by
truncate is not longer valid. So we are in big big troubles. 

I've add created inode's history tracer patch which spotted the issue.
See attachments.

--- Comment #1 from Dmitry Monakhov <dmonakhov@...nvz.org>  2010-04-21 15:41:13 ---
Created an attachment (id=26082)
 --> (https://bugzilla.kernel.org/attachment.cgi?id=26082)
debug patch against ext4.git/next + patches from bug #15792

The debug patch is rather ugly, but still useful.

--- Comment #2 from Dmitry Monakhov <dmonakhov@...nvz.org>  2010-04-21 15:55:15 ---
in given example both alloc and truncate tasks try to modify same path
path:  218:[1]33:108506
So ext4_ext_rm_leaf goes crazy because leaf node was modified.

Possible solutions:
1) Truncate_task: Drop path collected if ext4_ext_truncate_extend_restart
result in journal_restart, ant retry lookup.
2) Save current truncate path some where in inode, so get_block may check
   about conflicts and to let truncate task to know that path is no longer
valid.
   If so truncate task must goes back to (1).

--- Comment #3 from Dmitry Monakhov <dmonakhov@...nvz.org>  2010-04-22 04:36:10 ---
Proposed patch
http://patchwork.ozlabs.org/patch/50687/

BTW IMHO it is also reasonable to replace simple
printk("strange request..") to something more scary
like EXT4_ERROR_INODE(inode, "strange request..")

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