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Date:   Mon, 28 Sep 2020 21:46:56 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Nick Piggin <npiggin@gmail.com>, Hugh Dickins <hughd@google.com>,
        Peter Zijlstra <peterz@infradead.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] page_alloc: Fix freeing non-compound pages
Message-Id: <20200928214656.be4a0f29961589c074e518fa@linux-foundation.org>
In-Reply-To: <20200929011719.GD30994@casper.infradead.org>
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        <20200928180307.7573f3b6128b5e3007dfc9f0@linux-foundation.org>
        <20200929011719.GD30994@casper.infradead.org>
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On Tue, 29 Sep 2020 02:17:19 +0100 Matthew Wilcox <willy@infradead.org> wrote:

> On Mon, Sep 28, 2020 at 06:03:07PM -0700, Andrew Morton wrote:
> > On Sat, 26 Sep 2020 22:39:19 +0100 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:
> > 
> > > Here is a very rare race which leaks memory:
> > 
> > Not worth a cc:stable?
> 
> Yes, it probably should have been.

Have you a feeling for how often this occurs?

>  I just assume the stablebot will
> pick up anything that has a Fixes: tag.

We asked them not to do that for mm/ patches.  Crazy stuff was getting
backported.

> > >
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -4947,6 +4947,9 @@ void __free_pages(struct page *page, unsigned int order)
> > >  {
> > >  	if (put_page_testzero(page))
> > >  		free_the_page(page, order);
> > > +	else if (!PageHead(page))
> > > +		while (order-- > 0)
> > > +			free_the_page(page + (1 << order), order);
> > 
> > Well that's weird and scary looking.  `page' has non-zero refcount yet
> > we go and free random followon pages.  Methinks it merits an
> > explanatory comment?
> 
> Well, poot.  I lost that comment in the shuffling of patches.  In a
> different tree, I have:
> 
> @@ -4943,10 +4943,19 @@ static inline void free_the_page(struct page *page, unsi
> gned int order)
>                 __free_pages_ok(page, order);
>  }
>  
> +/*
> + * If we free a non-compound allocation, another thread may have a

"non-compound, higher-order", I suggest?

> + * speculative reference to the first page.  It has no way of knowing
> + * about the rest of the allocation, so we have to free all but the
> + * first page here.
> + */
>  void __free_pages(struct page *page, unsigned int order)
>  {
>         if (put_page_testzero(page))
>                 free_the_page(page, order);
> +       else if (!PageHead(page))
> +               while (order-- > 0)
> +                       free_the_page(page + (1 << order), order);
>  }
>  EXPORT_SYMBOL(__free_pages);
>  
> 
> Although I'm now thinking of making that comment into kernel-doc and
> turning it into advice to the caller rather than an internal note to
> other mm developers.

hm.  But what action could the caller take?  The explanatory comment
seems OK to me.