Re: O_DIRECT vs BLK_FEAT_STABLE_WRITES, was Re: [PATCH] btrfs: never trust the bio from direct IO

[Matthew Wilcox <willy@infradead.org>]

On Mon, Oct 20, 2025 at 03:59:33PM +0200, Jan Kara wrote:
> The idea was to bounce buffer the page we are writing back in case we spot
> a long-term pin we cannot just wait for - hence bouncing should be rare.
> But in this more general setting it is challenging to not bounce buffer for
> every IO (in which case you'd be basically at performance of RWF_DONTCACHE
> IO or perhaps worse so why bother?). Essentially if you hand out the real
> page underlying the buffer for the IO, all other attemps to do IO to that
> page have to block - bouncing is no longer an option because even with
> bouncing the second IO we could still corrupt data of the first IO once we
> copy to the final buffer. And if we'd block waiting for the first IO to
> complete, userspace could construct deadlock cycles - like racing IO to
> pages A, B with IO to pages B, A. So far I'm not sure about a sane way out
> of this...

There isn't one.  We might have DMA-mapped this page earlier, and so a
device could write to it at any time.  Even if we remove PTE write
permissions ...