linux-kernel - Re: [PATCH v2] x86: fix small LDT allocation for Xen

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Date:	Wed, 2 Sep 2015 18:59:22 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	David Vrabel <david.vrabel@...rix.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86: fix small LDT allocation for Xen


* Jan Beulich <JBeulich@...e.com> wrote:

> >> @@ -95,7 +95,7 @@ static void free_ldt_struct(struct ldt_s
> >>         if (ldt->size * LDT_ENTRY_SIZE > PAGE_SIZE)
> >>                 vfree(ldt->entries);
> >>         else
> >> -               kfree(ldt->entries);
> >> +               put_page(virt_to_page(ldt->entries));
> > 
> > FWIW, I'm not convinced this is or was correct.  Using free_page looks
> > a bit safer, and free_page does more than just put_page.
> 
> Actually I agree. put_page() is meant to be paired with get_page();
> __get_free_pages() is just misleading (i.e. doesn't imply a get_page())
> and instead is to be paired with free_pages(). Will do a v3 then.

So put_page() is a page cache primitive and should not be used outside of it.

As it happens, it is safe to put_page() a regularly allocated page as well, but 
it's a little bit slower than doing a free_page().

Thanks,

	Ingo
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