Re: [md PATCH 1/3] md/raid1: fix: IO can block resync indefinitely

[Shaohua Li <shli@kernel.org>]

On Wed, Nov 09, 2016 at 10:21:32AM +1100, Neil Brown wrote:
> While performing a resync/recovery, raid1 divides the
> array space into three regions:
>  - before the resync
>  - at or shortly after the resync point
>  - much further ahead of the resync point.
> 
> Write requests to the first or third do not need to wait.  Write
> requests to the middle region do need to wait if resync requests are
> pending.
> 
> If there are any active write requests in the middle region, resync
> will wait for them.
> 
> Due to an accounting error, there is a small range of addresses,
> between conf->next_resync and conf->start_next_window, where write
> requests will *not* be blocked, but *will* be counted in the middle
> region.  This can effectively block resync indefinitely if filesystem
> writes happen repeatedly to this region.

Good catch, thanks Neil! 
> As ->next_window_requests is incremented when the sector is before

I changed 'before' to 'after' when applying this patch

Thanks,
Shaohua