Re: [PATCH 1/1] fs/mpage.c: forgotten WRITE_SYNC in case of data integrity write

Tejun Heo
2014-03-14T14:11:49Z

Hello,

On Fri, Mar 14, 2014 at 11:07:04PM +0900, Roman Peniaev wrote:
> Seems the following message should be better:
> When data inegrity operation (sync, fsync, fdatasync calls) happens
> writeback control is set to WB_SYNC_ALL.
> In that case all write requests are marked with WRITE_SYNC, but on
> mpage writeback path
> WRITE_SYNC is missed. This patch fixes this.
>
> Is it ok, what do you think?

I think the description should make it clear that WRITE_SYNC is about
latency, not about integrity and we probably should add comments
explaining why we're using WRITE_SYNC for WB_SYNC_ALL (because there
probably is someone waiting).

> Also, could you please help me do understand how can I guarantee
> integrity in case of block device with big volatile
> cache and filesystem, which does not support REQ_FLUSH/FUA?

If a device has a volatile cache but doesn't support flush, it can't
guarantee integrity. There's no way for its user to determine or
force whether certain data is on non-volatile media. It's an
inherently broken device.

Thanks.

--
tejun