linux-kernel - Re: [PATCH 1/2] mm/memory: Do not populate page table entries beyond i

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Message-ID: <8379d8cb-aec5-44f7-a5f0-2356b8aaaf00@redhat.com>
Date: Tue, 21 Oct 2025 14:08:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Kiryl Shutsemau <kirill@...temov.name>,
 Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>,
 Matthew Wilcox <willy@...radead.org>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Hugh Dickins <hughd@...gle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Rik van Riel <riel@...riel.com>, Harry Yoo <harry.yoo@...cle.com>,
 Johannes Weiner <hannes@...xchg.org>, Shakeel Butt <shakeel.butt@...ux.dev>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Darrick J. Wong" <djwong@...nel.org>, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 Kiryl Shutsemau <kas@...nel.org>
Subject: Re: [PATCH 1/2] mm/memory: Do not populate page table entries beyond
 i_size.

On 21.10.25 08:35, Kiryl Shutsemau wrote:
> From: Kiryl Shutsemau <kas@...nel.org>

Subject: I'd drop the trailing "."

> 
> 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.

Right, there are some legacy oddities with shmem in that area (e.g., 
"within_size" vs. "always" THP allocation control).

Let me CC Hugh: the behavior for shmem seems to date back to 2016.

> 
> Fix filemap_map_pages() and finish_fault() to not install:
>    - PTEs beyond i_size;
>    - PMD mappings across i_size;

Makes sense to me.

[...]

> +++ b/mm/memory.c
> @@ -5480,6 +5480,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   	int type, nr_pages;
>   	unsigned long addr;
>   	bool needs_fallback = false;
> +	pgoff_t file_end = -1UL;
>   
>   fallback:
>   	addr = vmf->address;
> @@ -5501,8 +5502,14 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   			return ret;
>   	}
>   
> +	if (vma->vm_file) {
> +		struct inode *inode = vma->vm_file->f_mapping->host;

empty line pleae

> +		file_end = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> +	}
> +
>   	if (pmd_none(*vmf->pmd)) {
> -		if (folio_test_pmd_mappable(folio)) {
> +		if (folio_test_pmd_mappable(folio) &&
> +		    file_end >= folio_next_index(folio)) {
>   			ret = do_set_pmd(vmf, folio, page);
>   			if (ret != VM_FAULT_FALLBACK)
>   				return ret;
> @@ -5533,7 +5540,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   		if (unlikely(vma_off < idx ||
>   			    vma_off + (nr_pages - idx) > vma_pages(vma) ||
>   			    pte_off < idx ||
> -			    pte_off + (nr_pages - idx)  > PTRS_PER_PTE)) {
> +			    pte_off + (nr_pages - idx)  > PTRS_PER_PTE ||

While at it you could fix the double space before the ">".

> +			    file_end < folio_next_index(folio))) {
>   			nr_pages = 1;
>   		} else {
>   			/* Now we can set mappings for the whole large folio. */

Nothing else jumped at me.

-- 
Cheers

David / dhildenb