Re: [PATCHv2 1/2] mm/memory: Do not populate page table entries beyond i_size

[Kiryl Shutsemau]

On Mon, Oct 27, 2025 at 01:20:42AM -0700, Hugh Dickins wrote:
> On Thu, 23 Oct 2025, Kiryl Shutsemau 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;
> 
> Sorry for coming in late as usual, and complicating matters.
> 
> > 
> > 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")
> 
> ACK to restoring the correct POSIX behaviour to those filesystems
> which are being given large folios beyond EOF transparently,
> without any huge= mount option to permit it.
> 
> > Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> 
> But NAK to regressing the intentional behaviour of huge=always
> on shmem/tmpfs: the page size, whenever possible, is PMD-sized.  In
> 6.18-rc huge=always is currently (thanks to Baolin) behaving correctly
> again, as it had done for nine years: I insist we do not re-break it.
> 
> Andrew, please drop this version (and no need to worry about backports).
> 
> I'm guessing that yet another ugly shmem_file() or shmem_mapping()
> exception should be good enough - I doubt you need to consider the
> huge= option, just go by whether there is a huge folio already there -
> though that would have an implication for the following patch.
> 
> (But what do I mean by "huge folio" above?  Do I mean large or do
> I mean pmd_mappable?  It's the huge=always pmd_mappable folios I
> care not to break, the mTHPy ones can be argued either way.)

I assume you want the same exception for the second patch as well?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov