linux-kernel - Re: [PATCH 4/9] mm/rmap: change hugepage_add_new_anon

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Date:   Fri, 20 Jan 2023 12:45:38 -0800
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, songmuchun@...edance.com,
        mike.kravetz@...cle.com, jhubbard@...dia.com
Subject: Re: [PATCH 4/9] mm/rmap: change hugepage_add_new_anon_rmap to take in
 a folio

On 1/19/23 10:00 PM, Matthew Wilcox wrote:
> On Thu, Jan 19, 2023 at 01:14:41PM -0800, Sidhartha Kumar wrote:
>> @@ -5599,9 +5603,9 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>>   		goto out_release_all;
>>   	}
>>   
>> -	copy_user_huge_page(new_page, old_page, address, vma,
>> +	copy_user_huge_page(&new_folio->page, old_page, address, vma,
>>   			    pages_per_huge_page(h));
> 
> We have a folio_copy(), but it feels to me like we need a
> folio_copy_user() so that we can use copy_user_page() on machines
> with virtual caches.
> 
>> @@ -6176,6 +6186,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>>   	spinlock_t *ptl;
>>   	int ret = -ENOMEM;
>>   	struct page *page;
>> +	struct folio *folio = NULL;
>>   	int writable;
>>   	bool page_in_pagecache = false;
>>   
>> @@ -6251,12 +6262,15 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>>   		*pagep = NULL;
>>   	}
>>   
>> +	if (page)
>> +		folio = page_folio(page);
>> +
>>   	/*
>> -	 * The memory barrier inside __SetPageUptodate makes sure that
>> +	 * The memory barrier inside __folio_mark_uptodate makes sure that
>>   	 * preceding stores to the page contents become visible before
>>   	 * the set_pte_at() write.
>>   	 */
>> -	__SetPageUptodate(page);
>> +	__folio_mark_uptodate(folio);
> 

Hi Matthew,

In the snippet:

page = alloc_huge_page(dst_vma, dst_addr, 0);
if (IS_ERR(page)) {
	put_page(*pagep);
	ret = -ENOMEM;
	*pagep = NULL;
	goto out;
}
copy_user_huge_page(page, *pagep, dst_addr, dst_vma,
		pages_per_huge_page(h));

I thought the IS_ERR() call does not handle the NULL case and is a check 
for high memory addresses, and copy_user_huge_page() path does not seem 
to handle the NULL case as well but alloc_huge_page() can possibly 
return NULL so I was unsure about how to handle the folio conversion.

> I suggest that "page" can never be NULL or __SetPageUptodate() would
> have crashed.
>