Re: [PATCH] btrfs: remove btrfs_writepage_cow_fixup

[David Sterba <dsterba@suse.cz>]

On Tue, Jun 28, 2022 at 08:24:07AM +0800, Qu Wenruo wrote:
> On 2022/6/27 18:19, Jan Kara wrote:
> > On Sat 25-06-22 11:11:43, Christoph Hellwig wrote:
> >> On Fri, Jun 24, 2022 at 03:07:50PM +0200, Jan Kara wrote:
> >>> I'm not sure I get the context 100% right but pages getting randomly dirty
> >>> behind filesystem's back can still happen - most commonly with RDMA and
> >>> similar stuff which calls set_page_dirty() on pages it has got from
> >>> pin_user_pages() once the transfer is done. page_maybe_dma_pinned() should
> >>> be usable within filesystems to detect such cases and protect the
> >>> filesystem but so far neither me nor John Hubbart has got to implement this
> >>> in the generic writeback infrastructure + some filesystem as a sample case
> >>> others could copy...
> >>
> >> Well, so far the strategy elsewhere seems to be to just ignore pages
> >> only dirtied through get_user_pages.  E.g. iomap skips over pages
> >> reported as holes, and ext4_writepage complains about pages without
> >> buffers and then clears the dirty bit and continues.
> >>
> >> I'm kinda surprised that btrfs wants to treat this so special
> >> especially as more of the btrfs page and sub-page status will be out
> >> of date as well.
> >
> > I agree btrfs probably needs a different solution than what it is currently
> > doing if they want to get things right. I just wanted to make it clear that
> > the code you are ripping out may be a wrong solution but to a real problem.
> 
> IHMO I believe btrfs should also ignore such dirty but not managed by fs
> pages.
> 
> But I still have a small concern here.
> 
> Is it ensured that, after RDMA dirtying the pages, would we finally got
> a proper notification to fs that those pages are marked written?
> 
> If not, I would guess those pages would never got a chance to be written
> back.
> 
> If yes, then I'm totally fine to go the ignoring path.

This would work only for the higher level API where eg. RDMA notifies
the filesystem, but there's still the s390 case that is part of the
hardware architecture. The fixup worker is there as a safety for all
other cases, I'm not fine removing or ignoring it.