Re: [PATCHv3 1/2] mm/memory: Do not populate page table entries beyond i_size

[Kiryl Shutsemau]

On Mon, Oct 27, 2025 at 03:33:23PM -0700, Andrew Morton wrote:
> On Mon, 27 Oct 2025 11:56:35 +0000 Kiryl Shutsemau <kirill@xxxxxxxxxxxxx> wrote:
> 
> > From: Kiryl Shutsemau <kas@xxxxxxxxxx>
> > 
> > Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are
> > supposed to generate SIGBUS.
> > 
> > Recent changes attempted to fault in full folio where possible. They did
> > not respect i_size, which led to populating PTEs beyond i_size and
> > breaking SIGBUS semantics.
> > 
> > Darrick reported generic/749 breakage because of this.
> > 
> > However, the problem existed before the recent changes. With huge=always
> > tmpfs, any write to a file leads to PMD-size allocation. Following the
> > fault-in of the folio will install PMD mapping regardless of i_size.
> > 
> > Fix filemap_map_pages() and finish_fault() to not install:
> >   - PTEs beyond i_size;
> >   - PMD mappings across i_size;
> > 
> > Make an exception for shmem/tmpfs that for long time intentionally
> > mapped with PMDs across i_size.
> > 
> > Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> > Fixes: 19773df031bc ("mm/fault: try to map the entire file folio in finish_fault()")
> > Fixes: 357b92761d94 ("mm/filemap: map entire large folio faultaround")
> > Fixes: 01c70267053d ("fs: add a filesystem flag for THPs")
> 
> Multiple Fixes: are confusing.
> 
> We have two 6.18-rcX targets and one from 2020.  Are we asking people
> to backport this all the way back to 2020?  If so I'd suggest the
> removal of the more recent Fixes: targets.

Okay, fair enough.

> Also, is [2/2] to be backported?  The changelog makes it sound that way,
> but no Fixes: was identified?

Looking at split-on-truncate history, looks like this is the right
commit to point to:

Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios")

It moves split logic from shmem-specific to generic truncate.

As with the first patch, it will not be a trivial backport, but I am
around to help with this.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov