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

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Date:	Wed, 29 Apr 2009 13:57:27 +0200
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	"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

> 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.

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