Date:   Mon, 13 Feb 2023 15:50:03 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Alexandre Ghiti <alex@ghiti.fr>
Cc:     Marek Szyprowski <m.szyprowski@samsung.com>,
        "Vishal Moola (Oracle)" <vishal.moola@gmail.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [PATCH 2/2] mm/khugepaged: Convert release_pte_pages() to use
 folios
Message-ID: <Y+pcK41bagzzhgj+@casper.infradead.org>
References: <20230114001556.43795-1-vishal.moola@gmail.com>
 <20230114001556.43795-2-vishal.moola@gmail.com>
 <CGME20230213085312eucas1p2252fe07e3eadea5e2a80dfdd3aa96507@eucas1p2.samsung.com>
 <ff300770-afe9-908d-23ed-d23e0796e899@samsung.com>
 <3a3457b1-60bc-a97b-0042-432790d3bbbe@ghiti.fr>
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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
>