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

Matthew Wilcox
2024-09-19T04:36:17Z

On Wed, Sep 18, 2024 at 09:38:41PM -0600, Jens Axboe wrote:
> On 9/18/24 9:12 PM, Linus Torvalds wrote:
> > On Thu, 19 Sept 2024 at 05:03, Linus Torvalds
> > <torvalds@linux-foundation.org> 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)
>
> I kicked off a quick run with this on 6.9 with my debug patch as well,
> and it still fails for me... I'll double check everything is sane. For
> reference, below is the 6.9 filemap patch.
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 30de18c4fd28..88093e2b7256 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -883,6 +883,7 @@ noinline int __filemap_add_folio(struct address_space *mapping,
> if (order > folio_order(folio))
> xas_split_alloc(&xas, xa_load(xas.xa, xas.xa_index),
> order, gfp);
> + xas_reset(&xas);
> xas_lock_irq(&xas);
> xas_for_each_conflict(&xas, entry) {
> old = entry;

My brain is still mushy, but I think there is still a problem (both with
the simple fix for 6.9 and indeed with 6.10).

For splitting a folio, we have the folio locked, so we know it's not
going anywhere. The tree may get rearranged around it while we don't
have the xa_lock, but we're somewhat protected.

In this case we're splitting something that was, at one point, a shadow
entry. There's no struct there to lock. So I think we can have a
situation where we replicate 'old' (in 6.10) or xa_load() (in 6.9)
into the nodes we allocate in xas_split_alloc(). In 6.10, that's at
least guaranteed to be a shadow entry, but in 6.9, it might already be a
folio by this point because we've raced with something else also doing a
split.

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?