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 06:36, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> Probably xas_split_alloc() needs to just do the alloc, like the name
> says, and drop the 'entry' argument.  ICBW, but I think it explains
> what you're seeing?  Maybe it doesn't?

.. or we make the rule be that you have to re-check that the order and
the entry still matches when you do the actual xas_split()..

Like commit 6758c1128ceb does, in this case.

We do have another xas_split_alloc() - in the hugepage case - but
there we do have

                xas_lock(&xas);
                xas_reset(&xas);
                if (xas_load(&xas) != folio)
                        goto fail;

and the folio is locked over the whole sequence, so I think that code
is probably safe and guarantees that we're splitting with the same
details we alloc'ed.

                Linus