Re: [PATCH 1/2] dio: track and serialise unaligned direct IO

[Jan Kara <jack@suse.cz>]

On Mon 02-08-10 17:25:44, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> If we get two unaligned direct IO's to the same filesystem block
> that is marked as a new allocation (i.e. buffer_new), then both IOs
> will zero the portion of the block they are not writing data to. As
> a result, when the IOs complete there will be a portion of the block
> that contains zeros from the last IO to complete rather than the
> data that should be there.
> 
> This is easily manifested by qemu using aio+dio with an unaligned
> guest filesystem - every IO is unaligned and fileystem corruption is
> encountered in the guest filesystem. xfstest 240 (from Eric Sandeen)
> is also a simple reproducer.
> 
> To avoid this problem, track unaligned IO that triggers sub-block
> zeroing and check new incoming unaligned IO that require sub-block
> zeroing against that list. If we get an overlap where the start and
> end of unaligned IOs hit the same filesystem block, then we need to
> block the incoming IOs until the IO that is zeroing the block
> completes. The blocked IO can then continue without needing to do
> any zeroing and hence won't overwrite valid data with zeros.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
...
> +/*
> + * Add a filesystem block to the list of blocks we are tracking.
> + */
> +static void
> +dio_start_zero_block(struct dio *dio, sector_t zero_block)
> +{
> +	struct dio_zero_block *zb;
> +
> +	zb = kmalloc(sizeof(*zb), GFP_NOIO);
> +	if (!zb)
> +		return;
  Ho hum, so if the allocation fails, we will just silently corrupt the
data anyway? Not good I think.

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