Re: [PATCH] btrfs: remove btrfs_writepage_cow_fixup

[Jan Kara <jack@suse.cz>]

On Tue 28-06-22 10:29:00, Chris Mason wrote:
> I'd love a proper fix for this on the *_user_pages() side where
> page_mkwrite() style notifications are used all the time.  It's just a huge
> change, and my answer so far has always been that using btrfs mmap'd memory
> for this kind of thing isn't a great choice either way.

As Christoph wrote, it isn't a problem that you would not get a
page_mkwrite() notification. That happens just fine. But the problem is
that after that, the page can still get modified after you've removed all
writeable mappings of the page (e.g. by calling page_mkclean() in
clear_page_dirty_for_io()). And there's no way a kernel can provide further
notification for such writes because we've simply handed of the page
physical address to the HW to DMA into.

So the only viable solution is really for the filesystem to detect such
unprotectable pages if it cares and somehow deal with them (skip writeback,
use bounce pages, ...). The good news is that we already have
page_maybe_dma_pinned() call that at least allows detection of such
unprotectable pages.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR