Re: Known and unfixed active data loss bug in MM + XFS with large folios since Dec 2021 (any kernel from 6.1 upwards)

[Linus Torvalds]

On Thu, 19 Sept 2024 at 05:03, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> I think we should just do the simple one-liner of adding a
> "xas_reset()" to after doing xas_split_alloc() (or do it inside the
> xas_split_alloc()).

.. and obviously that should be actually *verified* to fix the issue
not just with the test-case that Chris and Jens have been using, but
on Christian's real PostgreSQL load.

Christian?

Note that the xas_reset() needs to be done after the check for errors
- or like Willy suggested, xas_split_alloc() needs to be re-organized.

So the simplest fix is probably to just add a

                        if (xas_error(&xas))
                                goto error;
                }
+               xas_reset(&xas);
                xas_lock_irq(&xas);
                xas_for_each_conflict(&xas, entry) {
                        old = entry;

in __filemap_add_folio() in mm/filemap.c

(The above is obviously a whitespace-damaged pseudo-patch for the
pre-6758c1128ceb state. I don't actually carry a stable tree around on
my laptop, but I hope it's clear enough what I'm rambling about)

               Linus