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

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Date:	Tue, 28 Apr 2009 14:35:56 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
CC:	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?

-Eric
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