Re: [PATCH] btrfs: remove btrfs_writepage_cow_fixup

[Christoph Hellwig <hch@lst.de>]

On Wed, Jun 29, 2022 at 11:45:47AM +0200, Jan Kara wrote:
> 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.

The bad news is that page_maybe_dma_pinned only accounts for FOLL_PIN
memory, and we till have a lot of users of FOLL_GET including direct I/O.

Now for FOLL_PIN I think most problems would be solved by
marking pages that are DMA pinned when writeback completes (that
is when PG_writeback is cleared) dirty again, and making sure the
equivalent of page_mkwrite is called for them again as well.  The
latter might be a bit ugly as PG_writeback could be cleared from
interrupt context, even if most modern file systems especially if
they do anything fancy like out of place writes or unwritten extents
defer it to a workqueue.

To reduce the overhead of this it would make sense to skip writing
these pages back at all unless it is a data integrity write.