Re: [PATCHv2 2/2] mm/truncate: Unmap large folio on split failure

[Kiryl Shutsemau]

On Thu, Oct 23, 2025 at 01:56:44PM -0700, Andrew Morton wrote:
> On Thu, 23 Oct 2025 10:32:51 +0100 Kiryl Shutsemau <kirill@xxxxxxxxxxxxx> wrote:
> 
> > Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are
> > supposed to generate SIGBUS.
> > 
> > This behavior might not be respected on truncation.
> > 
> > During truncation, the kernel splits a large folio in order to reclaim
> > memory. As a side effect, it unmaps the folio and destroys PMD mappings
> > of the folio. The folio will be refaulted as PTEs and SIGBUS semantics
> > are preserved.
> > 
> > However, if the split fails, PMD mappings are preserved and the user
> > will not receive SIGBUS on any accesses within the PMD.
> > 
> > Unmap the folio on split failure. It will lead to refault as PTEs and
> > preserve SIGBUS semantics.
> 
> This conflicts significantly with mm-hotfixes's
> https://lore.kernel.org/all/20251017013630.139907-1-ziy@xxxxxxxxxx/T/#u,
> whcih is cc:stable.
> 
> What do do here?

The patch below applies cleanly onto mm-everything.

Let me now if you want solve the conflict other way around. I can rebase
Zi's patch on top my patchset.