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

From: Ingo Molnar
Date: Wed Sep 02 2015 - 12:59:31 EST

* Jan Beulich <JBeulich@xxxxxxxx> 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().


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