Re: fall back from direct to buffered I/O when stable writes are required

[Keith Busch <kbusch@kernel.org>]

On Mon, Nov 03, 2025 at 01:21:11PM +0100, Christoph Hellwig wrote:
> On Mon, Nov 03, 2025 at 12:14:06PM +0100, Jan Kara wrote:
> > > Yes, it's pretty clear that the result in non-deterministic in what you
> > > get.  But that result still does not result in corruption, because
> > > there is a clear boundary ( either the sector size, or for NVMe
> > > optionally even a larger bodunary) that designates the atomicy boundary.
> > 
> > Well, is that boundary really guaranteed? I mean if you modify the buffer
> > under IO couldn't it happen that the DMA sees part of the sector new and
> > part of the sector old? I agree the window is small but I think the real
> > guarantee is architecture dependent and likely cacheline granularity or
> > something like that.
> 
> If you actually modify it: yes.  But I think Keith' argument was just
> about regular racing reads vs writes.

I was seeking documented behavior about concurrently modifying and
using any part of a host data buffer, so I look to storage specs. The
general guidance there aligns with "the reprecussions are your fault".
Linux DIO didn't say that, but I'm just saying there's precedence lower
down.

I'm not even sure how you handle the read side when multiple entities
are concurrently modifying the buffer. That has to be an application
bug even if bouncing it defeats the gaurd checks before the completion
overwrites the application's conflicting changes from the bounce buffer.