Re: writeback completion soft lockup BUG in folio_wake_bit()

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Mar 18, 2022 at 6:16 AM Jan Kara <jack@suse.cz> wrote:
>
> I agree with Dave that 'keep_towrite' thing is kind of self-inflicted
> damage on the ext4 side (we need to write out some blocks underlying the
> page but cannot write all from the transaction commit code, so we need to
> keep xarray tags intact so that data integrity sync cannot miss the page).
> Also it is no longer needed in the current default ext4 setup. But if you
> have blocksize < pagesize and mount the fs with 'dioreadlock,data=ordered'
> mount options, the hack is still needed AFAIK and we don't have a
> reasonable way around it.

I assume you meant 'dioread_lock'.

Which seems to be the default (even if 'data=ordered' is not).

Anyway - if it's not a problem for any current default setting, maybe
the solution is to warn about this case and turn it off?

IOW, we could simply warn about "data=ordered is no longer supported"
and turn it into data=journal.

Obviously *only* do this for the case of "blocksize < PAGE_SIZE".

If this ext4 thing is (a) obsolete and (b) causes VFS-level problems
that nobody else has, I really think we'd be much better off disabling
it than trying to work with it.

                 Linus