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

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Message-ID: <cba29ce0-24ef-4cef-9e54-74418c55c217@redhat.com>
Date: Wed, 25 Jun 2025 20:24:14 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>,
 "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 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 25.06.25 20:00, Matthew Wilcox wrote:
> 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.

Not special to anon memory I think ... :)

Only the offset within a page is okay to derive (existing code).

-- 
Cheers,

David / dhildenb