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

[Hugh Dickins]

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.)

Hugh