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

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Date:   Fri, 20 Jan 2023 14:05:57 -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 1/20/23 1:17 PM, Matthew Wilcox wrote:
> On Fri, Jan 20, 2023 at 12:45:38PM -0800, Sidhartha Kumar wrote:
>>>> @@ -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'm not sure how alloc_huge_page() can return NULL.  It seems like it
> returns ERR_PTR(-ENOSPC) or ERR_PTR(-ENOMEM) if it cannot allocate memory?
> 
I see now, I agree that page cannot be NULL at the return from 
alloc_huge_page, I will make that change in v2.

Thanks,
Sidhartha Kumar