Re: [PATCH] huge_memory: return -EINVAL in folio split functions when THP is disabled

[Kiryl Shutsemau <kirill@shutemov.name>]

On Tue, Sep 02, 2025 at 02:15:42PM +0200, Pankaj Raghav wrote:
> 
> On 9/2/25 13:22, Kiryl Shutsemau wrote:
> > On Tue, Sep 02, 2025 at 10:40:36AM +0200, Pankaj Raghav (Samsung) wrote:
> >> From: Pankaj Raghav <p.raghav@samsung.com>
> >>
> >> split_huge_page_to_list_[to_order](), split_huge_page() and
> >> try_folio_split() return 0 on success and error codes on failure.
> >>
> >> When THP is disabled, these functions return 0 indicating success even
> >> though an error code should be returned as it is not possible to split a
> >> folio when THP is disabled.
> > 
> > Other view is that the page is already split therefore nop.
> > 
> >> Make all these functions return -EINVAL to indicate failure instead of
> >> 0.
> >>
> >> This issue was discovered while experimenting enabling large folios
> >> without THP and found that returning 0 in these functions is resulting in
> >> undefined behavior in truncate operations. This change fixes the issue.
> > 
> > Could you elaborate on the undefined behaviour? I don't see it.
> > 
> > If you argue that this code should not be reachable on !THP config, add
> > WARN() there. But I don't see a value.
> 
> Little bit of context:
> 
> I started investigating what it takes to remove large folio dependency on THP[1][2]
> 
> I have some non-upstream changes which enables Large block size (therefore it uses large folios) on
> systems with !CONFIG_THP.
> 
> I was hitting a weird stale content read error and finally ended up with this fix.
> 
> I thought this is a self-contained patch that can already be upstream. My argument is not that this
> should not be reachable, but returning -EINVAL will do the right thing instead of returning 0, which
> means success.

Okay, makes sense.

In THP=y case, __folio_split() also returns -EINVAL for !large folios,
but it is not very explicit:

	if (new_order >= folio_order(folio))
		return -EINVAL;

In THP=y, we also issue warning:

	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);

Makes sense to do the same here for THP=n. It might help to catch cases
we do not see with THP=y, like getting non-THP large folios here.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov