From: Linus Torvalds <torvalds@linux-foundation.org>
To: Hugh Dickins <hughd@google.com>
Cc: "Dave Hansen" <dave.hansen@linux.intel.com>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
"Kees Cook" <keescook@google.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Ingo Molnar" <mingo@kernel.org>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"Jürgen Groß" <jgross@suse.com>,
"Josh Poimboeuf" <jpoimboe@redhat.com>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"Peter Zijlstra" <peterz@infradead.org>,
"Borislav Petkov" <bp@alien8.de>,
"Andrew Lutomirski" <luto@kernel.org>,
"Andi Kleen" <ak@linux.intel.com>
Subject: Re: [PATCH 3/7] x86/mm/init: pass unconverted symbol addresses to free_init_pages()
Date: Sat, 4 Aug 2018 10:31:29 -0700 [thread overview]
Message-ID: <CA+55aFzd-F65GVBq8a-vTaU4G5UwddDcOdcd3SV9YVdB6qDGdg@mail.gmail.com> (raw)
In-Reply-To: <alpine.LSU.2.11.1808031650050.4436@eggly.anvils>
On Fri, Aug 3, 2018 at 5:19 PM Hugh Dickins <hughd@google.com> wrote:
>
> I thought that virt_to_page() only works on virtual addresses
> in the direct map
You're right that virt_to_page() does not work on any _actual_ virtual
mappings (ie no user pages, and no vmalloc() pages etc). It does not
follow page tables at all.
And on 32-bit, it literally ends up doing (see __phys_addr_nodebug()) a simple
#define __phys_addr_nodebug(x) ((x) - PAGE_OFFSET)
However, on x86-64, we have *two* cases of direct mappings: we have
the one at __START_KERNEL_map, and we have the one at PAGE_OFFSET.
And virt_to_page() handles both of those direct mappings.
Annoying? Yes. And it has caused bugs in the past. And I entirely
forget why we needed it on x86-64.
[ Goes around and rummages ]
Oh, never mind, looking around reminded me why: we want to map the
kernel text in the top 31 bits, so that we can use the faster
-mcmodel=kernel because all symbols fit in sign-extended 32 bits.
Maybe there was some other reason too, but I think that's it.
Linus
next prev parent reply other threads:[~2018-08-04 17:31 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-02 22:58 [PATCH 0/7] [v2] x86/mm/pti: close two Meltdown leaks with Global kernel mapping Dave Hansen
2018-08-02 22:58 ` [PATCH 1/7] x86/mm/pti: clear Global bit more aggressively Dave Hansen
2018-08-05 20:30 ` [tip:x86/pti] x86/mm/pti: Clear " tip-bot for Dave Hansen
2018-08-02 22:58 ` [PATCH 2/7] mm: allow non-direct-map arguments to free_reserved_area() Dave Hansen
2018-08-05 20:31 ` [tip:x86/pti] mm: Allow " tip-bot for Dave Hansen
2018-08-02 22:58 ` [PATCH 3/7] x86/mm/init: pass unconverted symbol addresses to free_init_pages() Dave Hansen
2018-08-04 0:18 ` Hugh Dickins
2018-08-04 17:31 ` Linus Torvalds [this message]
2018-08-04 18:23 ` Hugh Dickins
2018-08-05 6:11 ` Andi Kleen
2018-08-05 20:31 ` [tip:x86/pti] x86/mm/init: Pass " tip-bot for Dave Hansen
2018-08-02 22:58 ` [PATCH 4/7] x86/mm/init: add helper for freeing kernel image pages Dave Hansen
2018-08-05 20:32 ` [tip:x86/pti] x86/mm/init: Add " tip-bot for Dave Hansen
2018-08-02 22:58 ` [PATCH 5/7] x86/mm/init: remove freed kernel image areas from alias mapping Dave Hansen
2018-08-04 0:35 ` Hugh Dickins
2018-08-04 21:38 ` Andy Lutomirski
2018-08-06 15:17 ` Dave Hansen
2018-08-05 20:32 ` [tip:x86/pti] x86/mm/init: Remove " tip-bot for Dave Hansen
2018-08-06 20:21 ` [tip:x86/pti-urgent] " tip-bot for Dave Hansen
2018-08-02 22:58 ` [PATCH 6/7] x86/mm/pageattr: pass named flag instead of 0/1 Dave Hansen
2018-08-05 20:09 ` Thomas Gleixner
2018-08-06 15:09 ` Dave Hansen
2018-08-02 22:58 ` [PATCH 7/7] x86/mm/pageattr: Remove implicit NX behavior Dave Hansen
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