linux-ext4 - Re: [RFC PATCH] mark buffer_head mapping preallocate area as new during write

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Date:	Wed, 29 Apr 2009 20:13:21 +0200
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Jan Kara <jack@...e.cz>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Theodore Tso <tytso@....edu>, Mingming Cao <cmm@...ibm.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [RFC PATCH] mark buffer_head mapping preallocate area as new
	during write_begin with delayed allocation

On Wed 29-04-09 09:08:05, Eric Sandeen wrote:
> Jan Kara wrote:
> >> Aneesh Kumar K.V wrote:
> >>> On Tue, Apr 28, 2009 at 01:00:47PM -0400, Theodore Tso wrote:
> >>>> On Tue, Apr 28, 2009 at 10:05:54PM +0530, Aneesh Kumar K.V wrote:
> >> ...
> >>>>>> The other problem seems to be in the case of a delayed allocation
> >>>>>> write, where we return a buffer_head which is marked new, and this
> >>>>>> causes block_prepare_write() to call unmap_underlying_metadata(dev, 0).
> >>>>> Not just that. On block allocation we are not calling
> >>>>> unmap_underlying_metadata(dev, blocknumber) for delayed allocated
> >>>>> blocks. That would imply file corruption.
> >>>> I don't think I'm following you .  If we write into block that was
> >>>> delayed allocated.  Are you saying we might get in trouble of the
> >>>> delayed allocation block is mmap'ed in?
> >>> We allocate blocks for delayed buffer during writepage. Now we need to
> >>> make sure after getting the blocks we drop the old buffer_head mapping
> >>> that we may have with this particular block attached to the block
> >>> device. That is done by calling unmap_underlying_metadata. Now the
> >>> current code doesn't call unmap_underlying_metadata for delayed
> >>> allocated blocks. That would mean we can see corrupt files if old
> >>> buffer_head mapping gets synced to disk AFTER we write the new
> >>> buffer_head mapping.
> >>
> >> Talking w/ Aneesh on IRC, I don't see how we can have stray dirty
> >> mappings lying around for this block device unless someone is writing
> >> directly to the mounted block device, which I don't think is ever
> >> considered safe ...
> >>
> >> I'm not quite sure what the call to __unmap_underlying_blocks() in
> >> mpage_da_map_blocks() is for, I guess?
> >   For ext3 / ext4 I think we don't need unmap_underlying_blocks() since
> > before we reallocate a block, we make sure that the transaction freeing
> > the block is committed and clear all dirty bits from freed blocks.
> >   But for more careless filesystems, if they reallocate metadata block
> > as a data block and don't clear the dirty bit in blockdev mapping,
> > unmap_underlying_blocks() does it for them.
> 
> That's what I thought - so I was wondering why we have specific calls to
> this in ext4:
> 
> mpage_da_map_blocks
> 	__unmap_underlying_blocks
> 		for (i = 0; i < blocks; i++)
> 			unmap_underlying_metadata
  Hmm, OK. So maybe change it warn on dirty blockdev buffer and if the warning
does not trigger we can believe that our theory is right ;).

									Honza

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