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

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

[Zhengyuan Liu]

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.

Signed-off-by: Zhengyuan Liu <liuzhengyuan@kylinos.cn>
---
 drivers/md/raid5-cache.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 998ea00..981f855 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -1156,6 +1156,7 @@ create:
 	if (create_super) {
 		log->last_cp_seq = prandom_u32();
 		cp = 0;
+		r5l_log_write_empty_meta_block(log, cp, log->last_cp_seq);
 		/*
 		 * Make sure super points to correct address. Log might have
 		 * data very soon. If super hasn't correct log tail address,
-- 
2.7.4

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

[Shaohua Li]

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!