Re: [PATCH -RFC 0/2] mm/ext4: avoid data corruption when extending DIO write race with buffered read

Christoph Hellwig
2023-12-06T09:02:53Z

On Wed, Dec 06, 2023 at 07:35:35PM +1100, Dave Chinner wrote:
> Mixing overlapping buffered read with direct writes - especially partial block
> extending DIO writes - is a recipe for data corruption. It's not a
> matter of if, it's a matter of when.
>
> Fundamentally, when you have overlapping write IO involving DIO, the
> result of the overlapping IOs is undefined. One cannot control
> submission order, the order that the overlapping IO hit the
> media, or completion ordering that might clear flags like unwritten
> extents. The only guarantee that we give in this case is that we
> won't expose stale data from the disk to the user read.

Btw, one thing we could do to kill these races forever is to track if
there are any buffered openers for an inode and just fall back to
buffered I/O for that case. With that and and inode_dio_wait for
when opening for buffered I/O we'd avoid the races an various crazy
workarounds entirely.

nfs and ocfs2 do (or did, I haven't checked for a while) something
like that.