Re: [PATCH] block: flush the disk cache on BLKFLSBUF

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jun 27, 2023 at 05:31:12PM +0200, Mikulas Patocka wrote:
> Marc Smith reported a bug where he wrote to the dm-writecache target using 
> O_DIRECT, then reset the machine without proper shutdown and the freshly 
> written data were lost. It turned out that he didn't use the fsync or 
> fdatasync syscall (and dm-writecache makes its metadata persistent on a 
> FLUSH bio).

Which so far is expected.  Even with O_DIRECT you need O_(D)SYNC or
fsync/fdatasync to persist data.

> When I was analyzing this issue, it turned out that there is no easy way 
> how to send the FLUSH bio to a block device from a command line.

xfs_io -c fsync /dev/foo

> The "blockdev --flushbufs" command also doesn't send the FLUSH bio, but I 
> would expect it to send it. Without sending the FLUSH bio, "blockdev 
> --flushbufs" doesn't really guarantee anything.

I wouldn't expect it.  It's a really weird legacy thing that calls
back up into the file system, but only if it sets s_bdev to this
device.  I don't think we should add new users of it that overload the
semantics.