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

[Christoph Hellwig <hch@lst.de>]

On Fri, Nov 14, 2025 at 10:29:39AM +0100, Kevin Wolf wrote:
> Right, but since this is direct I/O and the approach with only declaring
> I/O from the page cache safe without a bounce buffer means that RAID has
> to use a bounce buffer here anyway (with or without PI), doesn't this
> automatically solve it?
>
> So if it's only PI, it's the problem of userspace, and if you add RAID
> on top, then the normal rules for RAID apply. (And that the buffer
> doesn't get modified and PI doesn't become invalid until RAID does its
> thing is still a userspace problem.)

Well, only if we have different levels of I/O stability guarantees:

Level 0
  - trusted caller guarantees pages are stable (buffered I/O,
    in-kernel direct I/O callers that control the buffer)

Level 1:
  - untrusted caller declares the pages are stable
    (direct I/O with PI)

Level 2:
  - no one guarantees nothing
    (other direct I/O directly or indirectly fed from userspace)

PI formatted devices would only bounce for 1, parity would bounce for
1 and 2.  Software checksums could probably get away with only 1,
although 2 would feel safer.