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

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Date:   Fri, 20 Jan 2023 21:17:14 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Sidhartha Kumar <sidhartha.kumar@...cle.com>
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 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?