Re: writeback completion soft lockup BUG in folio_wake_bit()

[Matthew Wilcox <willy@infradead.org>]

On Thu, Mar 17, 2022 at 12:26:35PM -0700, Linus Torvalds wrote:
> On Thu, Mar 17, 2022 at 8:04 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > So how about we do something like this:
> >
> >  - Make folio_start_writeback() and set_page_writeback() return void,
> >    fixing up AFS and NFS.
> >  - Add a folio_wait_start_writeback() to use in the VFS
> >  - Remove the calls to set_page_writeback() in the filesystems
> 
> That sounds lovely, but it does worry me a bit. Not just the odd
> 'keepwrite' thing, but also the whole ordering between the folio bit
> and the tagging bits. Does the ordering possibly matter?

I wouldn't change the ordering of setting the xarray bits and the
writeback flag; they'd just be set a little earlier.  It'd all be done
while the page was still locked.  But you're right, there's lots of
subtle interactions here.

> That whole "xyz_writeback_keepwrite()" thing seems odd. It's used in
> only one place (the folio version isn't used at all):
> 
>   ext4_writepage():
> 
>      ext4_walk_page_buffers() fails:
>                 redirty_page_for_writepage(wbc, page);
>                 keep_towrite = true;
>       ext4_bio_write_page().
> 
> which just looks odd. Why does it even try to continue to do the
> writepage when the page buffer thing has failed?
> 
> In the regular write path (ie ext4_write_begin()), a
> ext4_walk_page_buffers() failure is fatal or causes a retry). Why is
> ext4_writepage() any different? Particularly since it wants to keep
> the page dirty, then trying to do the writeback just seems wrong.
> 
> So this code is all a bit odd, I suspect there are decades of "people
> continued to do what they historically did" changes, and it is all
> worrisome.

I found the commit: 1c8349a17137 ("ext4: fix data integrity sync in
ordered mode").  Fortunately, we have a documented test for this,
generic/127, so we'll know if we've broken it.