linux-kernel - Re: [PATCH 2/2] mm/khugepaged: Convert release_pte

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Date:   Mon, 13 Feb 2023 15:50:03 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Alexandre Ghiti <alex@...ti.fr>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 2/2] mm/khugepaged: Convert release_pte_pages() to use
 folios

On Mon, Feb 13, 2023 at 04:28:05PM +0100, Alexandre Ghiti wrote:
> The issue lies here: before using pteval in pfn_folio(), we should test it.
> The following patch fixes the issue for me:

Thanks for debugging it.  I'd rather see this written as ...

		pte_t pteval = *_pte;
+		unsigned long pfn;

+		if (pte_none(pteval))
+			continue;
+		pfn = pte_pfn(pteval);
+		if (is_zero_pfn(pfn))
+			continue;
+		folio = pfn_folio(pfn);
+		if (folio_test_large(folio))
+			continue;
		release_pte_folio(folio);

makes sense?

> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index eb38bd1b1b2f..fef3414b481b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -514,10 +514,12 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
>         while (--_pte >= pte) {
>                 pte_t pteval = *_pte;
> 
> -               folio = pfn_folio(pte_pfn(pteval));
> -               if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)) &&
> -                               !folio_test_large(folio))
> -                       release_pte_folio(folio);
> +               if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval))) {
> +                       folio = pfn_folio(pte_pfn(pteval));
> +
> +                       if (!folio_test_large(folio))
> +                               release_pte_folio(folio);
> +               }
>         }
> 
>         list_for_each_entry_safe(folio, tmp, compound_pagelist, lru) {
> 
> 
> @Marek: could you give it a try?
> 
> I can send a separate patch if needed, let me know.
> 
> Thanks,
> 
> Alex
> 
> 
> > >    		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)) &&
> > > -				!PageCompound(page))
> > > -			release_pte_page(page);
> > > +				!folio_test_large(folio))
> > > +			release_pte_folio(folio);
> > >    	}
> > > -	list_for_each_entry_safe(page, tmp, compound_pagelist, lru) {
> > > -		list_del(&page->lru);
> > > -		release_pte_page(page);
> > > +	list_for_each_entry_safe(folio, tmp, compound_pagelist, lru) {
> > > +		list_del(&folio->lru);
> > > +		release_pte_folio(folio);
> > >    	}
> > >    }
> > Best regards
>