On Thu, Sep 22, 2022 at 01:29:35PM +0200, Jan Kara wrote:

quoted

This rule would mostly work, as long as we can relax it in some cases, to
allow pinning of both source and dest pages, instead of just destination
pages, in some cases. In particular, bio_release_pages() has lost all
context about whether it was a read or a write request, as far as I can
tell. And bio_release_pages() is the primary place to unpin pages for
direct IO.

Well, we already do have BIO_NO_PAGE_REF bio flag that gets checked in
bio_release_pages(). I think we can easily spare another bio flag to tell
whether we need to unpin or not. So as long as all the pages in the created
bio need the same treatment, the situation should be simple.

Yes.  Incidentally, the same condition is already checked by the creators
of those bio - see the assorted should_dirty logics.

While we are at it - how much of the rationale around bio_check_pages_dirty()
doing dirtying is still applicable with pinning pages before we stick them
into bio?  We do dirty them before submitting bio, then on completion
bio_check_pages_dirty() checks if something has marked them clean while
we'd been doing IO; if all of them are still dirty we just drop the pages
(well, unpin and drop), otherwise we arrange for dirty + unpin + drop
done in process context (via schedule_work()).  Can they be marked clean by
anyone while they are pinned?  After all, pinning is done to prevent
writeback getting done on them while we are modifying the suckers...