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

[David Hildenbrand]

On 23.10.25 11:32, 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;
>
> 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: 800d8c63b2e9 ("shmem: add huge pages support")
> Reported-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> ---

Some of the code in here might deserve some cleanups IMHO :)

[...]

>   	addr0 = addr - start * PAGE_SIZE;
>   	if (folio_within_vma(folio, vmf->vma) &&
> -	    (addr0 & PMD_MASK) == ((addr0 + folio_size(folio) - 1) & PMD_MASK)) {
> +	    (addr0 & PMD_MASK) == ((addr0 + folio_size(folio) - 1) & PMD_MASK) &&

Isn't this just testing whether addr0 is aligned to folio_size(folio)? 
(given that we don't support folios > PMD_SIZE), like

	IS_ALIGNED(addr0, folio_size(folio))

Anyhow, unrelated to this patch ...



Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers

David / dhildenb