linux-ext4 - Re: writeback completion soft lockup BUG in folio_wake

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Date:   Fri, 18 Mar 2022 11:56:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Brian Foster <bfoster@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Namjae Jeon <namjae.jeon@...sung.com>,
        Ashish Sangwan <a.sangwan@...sung.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: writeback completion soft lockup BUG in folio_wake_bit()

On Fri, Mar 18, 2022 at 6:16 AM Jan Kara <jack@...e.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