linux-kernel - Re: [RFC PATCH 1/3] mm/memory.c: convert __copy_remote_vm

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Message-ID: <aFw5NuCbgJOBmG97@casper.infradead.org>
Date: Wed, 25 Jun 2025 19:00:22 +0100
From: Matthew Wilcox <willy@...radead.org>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	Jordan Rome <linux@...danrome.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/3] mm/memory.c: convert __copy_remote_vm_str() to
 folios

On Wed, Jun 25, 2025 at 10:48:39AM -0700, Vishal Moola (Oracle) wrote:
> +++ b/mm/memory.c
> @@ -6820,9 +6820,10 @@ static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr,
>  	}
>  
>  	while (len) {
> -		int bytes, offset, retval;
> +		int bytes, folio_offset, page_offset retval;

offset_in_folio() returns a size_t so that we can support folios larger
than 2GB (which is a real possibility here; hugetlbfs might end up with
a 16GB folio on some architectures).

> @@ -6837,17 +6838,20 @@ static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr,
>  			goto out;
>  		}
>  
> +		folio = page_folio(page);
>  		bytes = len;
> -		offset = addr & (PAGE_SIZE - 1);
> -		if (bytes > PAGE_SIZE - offset)
> -			bytes = PAGE_SIZE - offset;
> +		folio_offset = offset_in_folio(folio, addr);

Umm.  Not sure this is safe.  A folio might be mapped misaligned, so
'addr' might not give you the right offset within the folio.  I think
you might need to use addr - (vma->vm_pgoff << PAGE_SHIFT).  But I'd
defer to others here ... particularly when it comes to anonymous folios.