Re: [PATCH] md/raid5: write an empty meta-block when creating log super-block

[Shaohua Li <shli@kernel.org>]

On Mon, Oct 24, 2016 at 04:15:59PM +0800, Zhengyuan Liu wrote:
> If superblock points to an invalid meta block, r5l_load_log will set
> create_super with true and create an new superblock, this runtime path
> would always happen if we do no writing I/O to this array since it was
> created. Writing an empty meta block could avoid this unnecessary
> action at the first time we created log superblock.
> 
> Another reason is for the corretness of log recovery. Currently we have
> bellow code to guarantee log revocery to be correct.
> 
>         if (ctx.seq > log->last_cp_seq + 1) {
>                 int ret;
> 
>                 ret = r5l_log_write_empty_meta_block(log, ctx.pos, ctx.seq + 10);
>                 if (ret)
>                         return ret;
>                 log->seq = ctx.seq + 11;
>                 log->log_start = r5l_ring_add(log, ctx.pos, BLOCK_SECTORS);
>                 r5l_write_super(log, ctx.pos);
>         } else {
>                 log->log_start = ctx.pos;
>                 log->seq = ctx.seq;
>         }
> 
> If we just created a array with a journal device, log->log_start and
> log->last_checkpoint should all be 0, then we write three meta block
> which are valid except mid one and supposed crash happened. The ctx.seq
> would equal to log->last_cp_seq + 1 and log->log_start would be set to
> position of mid invalid meta block after we did a recovery, this will
> lead to problems which could be avoided with this patch.

This would be very unlikely, but better to fix. Applied, thanks!