linux-kernel - Re: [PATCH 1/2] sparc: Fix handling of page table constructor failure

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Message-ID: <20201125121037.GJ4327@casper.infradead.org>
Date:   Wed, 25 Nov 2020 12:10:37 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, davem@...emloft.net, rppt@...nel.org,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 1/2] sparc: Fix handling of page table constructor failure

On Wed, Nov 25, 2020 at 09:43:15AM +0100, David Hildenbrand wrote:
> On 25.11.20 04:46, Matthew Wilcox (Oracle) wrote:
> > The page has just been allocated, so its refcount is 1.  free_unref_page()
> > is for use on pages which have a zero refcount.  Use __free_page()
> > like the other implementations of pte_alloc_one().
> > 
> > Fixes: 1ae9ae5f7df7 ("sparc: handle pgtable_page_ctor() fail")
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > ---
> >  arch/sparc/mm/init_64.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/sparc/mm/init_64.c b/arch/sparc/mm/init_64.c
> > index 96edf64d4fb3..182bb7bdaa0a 100644
> > --- a/arch/sparc/mm/init_64.c
> > +++ b/arch/sparc/mm/init_64.c
> > @@ -2894,7 +2894,7 @@ pgtable_t pte_alloc_one(struct mm_struct *mm)
> >  	if (!page)
> >  		return NULL;
> >  	if (!pgtable_pte_page_ctor(page)) {
> > -		free_unref_page(page);
> > +		__free_page(page);
> >  		return NULL;
> >  	}
> >  	return (pte_t *) page_address(page);
> > 
> 
> I wonder if reusing __pte_alloc_one() - e.g., internally - would be even
> cleaner.

It's really awkward to do because pgtable_t is defined differently.
The clean thing to do would be:

--- arch/sparc/include/asm/page_64.h
-typedef pte_t *pgtable_t;
+typedef struct page *pgtable_t;

and then do all the other changes that would require.

But that feels like a lot more work than appropriate to fix this
unlikely bug.