On Mon 02-08-10 17:25:44, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
...
> +/*
> + * 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@xxxxxxx>
SUSE Labs, CR
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