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

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 14, 2023 at 02:56:04PM +0100, Jan Kara wrote:
> On Mon 13-02-23 01:59:28, Christoph Hellwig wrote:
> > Eww.  The block bounc code really needs to go away, so a new user
> > makes me very unhappy.
> > 
> > But independent of that I don't think this is enough anyway.  Just
> > copying the data out into a new page in the block layer doesn't solve
> > the problem that this page needs to be tracked as dirtied for fs
> > accounting.  e.g. every time we write this copy it needs space allocated
> > for COW file systems.
> 
> Right, I forgot about this in my RFC. My original plan was to not clear the
> dirty bit in clear_page_dirty_for_io() even for WB_SYNC_ALL writeback when
> we do writeback the page and perhaps indicate this in the return value of
> clear_page_dirty_for_io() so that the COW filesystem can keep tracking this
> page as dirty.

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.

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com