From: Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH] ext3_getblk should handle HOLE correctly
Date: Thu, 7 Sep 2006 12:55:55 -0700
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To: Badari Pulavarty <pbadari@us.ibm.com>
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On Thu, 07 Sep 2006 12:22:27 -0700
Badari Pulavarty <pbadari@us.ibm.com> wrote:

> On Thu, 2006-09-07 at 11:45 -0700, Andrew Morton wrote:
> > On Wed, 06 Sep 2006 10:39:06 -0700
> > Badari Pulavarty <pbadari@us.ibm.com> wrote:
> > 
> > > Hi Andrew,
> > > 
> > > Its been reported that ext3_getblk() is not doing the right thing
> > > and triggering following WARN():
> > > 
> > > BUG: warning at fs/ext3/inode.c:1016/ext3_getblk()
> > >  <c01c5140> ext3_getblk+0x98/0x2a6  <c03b2806> md_wakeup_thread
> > > +0x26/0x2a
> > >  <c01c536d> ext3_bread+0x1f/0x88  <c01cedf9> ext3_quota_read+0x136/0x1ae
> > >  <c018b683> v1_read_dqblk+0x61/0xac  <c0188f32> dquot_acquire+0xf6/0x107
> > >  <c01ceaba> ext3_acquire_dquot+0x46/0x68  <c01897d4> dqget+0x155/0x1e7
> > >  <c018a97b> dquot_transfer+0x3e0/0x3e9  <c016fe52> dput+0x23/0x13e
> > >  <c01c7986> ext3_setattr+0xc3/0x240  <c0120f66> current_fs_time
> > > +0x52/0x6a
> > >  <c017320e> notify_change+0x2bd/0x30d  <c0159246> chown_common+0x9c/0xc5
> > >  <c02a222c> strncpy_from_user+0x3b/0x68  <c0167fe6> do_path_lookup
> > > +0xdf/0x266
> > >  <c016841b> __user_walk_fd+0x44/0x5a  <c01592b9> sys_chown+0x4a/0x55
> > >  <c015a43c> vfs_write+0xe7/0x13c  <c01695d4> sys_mkdir+0x1f/0x23
> > >  <c0102a97> syscall_call+0x7/0xb 
> > > 
> > > Looking at the code, it looks like its not handle HOLE correctly.
> > > It ends up returning -EIO.
> > 
> > Strange.  The fs should be spewing these warnings all over the place.  For
> > some reason this code is hard to trigger.  Why??
> 
> I guess - ext3_getblk() mostly used by ext3_bread() and most callers 
> to it would be reading already allocated block.

OK.

> > 
> > > -	if (err == 1) {
> > > +	/*
> > > +	 * ext3_get_blocks_handle() returns number of blocks
> > > +	 * mapped. 0 in case of a HOLE.
> > > +	 */
> > > +	if (err > 0) {
> > >  		err = 0;
> > > -	} else if (err >= 0) {
> > > -		WARN_ON(1);
> > > -		err = -EIO;
> > >  	}
> > 
> > That removes the warning if ext3_get_blocks_handle() returned a positive
> > number greater than one.  And it looks like we still need debugging support
> > in this area.
> 
> I am not sure why we need it ? All we care about is one block. If
> ext3_get_blocks_handle() returns more than one (which it shouldn't) -

The operative part is "which it shouldn't".  This code is fairly fresh, and
ext3 is paranoid.  Once it's all settled down then after a year or so we
might decide that the debugging code is no longer needed.

Then again, we have plenty of five-year-old assertions in there..