Re: [PATCH 4/5] block: Add support for bouncing pinned pages

[Christoph Hellwig <hch@infradead.org>]

On Wed, Feb 15, 2023 at 03:59:52PM +1100, Dave Chinner wrote:
> I don't think this works, especially if the COW mechanism relies on
> delayed allocation to prevent ENOSPC during writeback. That is, we
> need a write() or page fault (to run ->page_mkwrite()) after every
> call to folio_clear_dirty_for_io() in the writeback path to ensure
> that new space is reserved for the allocation that will occur
> during a future writeback of that page.
> 
> Hence we can't just leave the page dirty on COW filesystems - it has
> to go through a clean state so that the clean->dirty event can be
> gated on gaining the space reservation that allows it to be written
> back again.

Exactly.  Although if we really want we could do the redirtying without
formally moving to a clean state, but it certainly would require special
new code to the same steps as if we were redirtying.

Which is another reason why I'd prefer to avoid all that if we can.
For that we probably need an inventory of what long term pins we have
in the kernel tree that can and do operate on shared file mappings,
and what kind of I/O semantics they expect.