Re: [Bug 10732] REGRESSION: 2.6.26-rc2-git4: X server failed startonX61s laptop

From: Jeremy Fitzhardinge
Date: Tue May 20 2008 - 02:32:20 EST

Linus Torvalds wrote:
On Mon, 19 May 2008, Hugh Dickins wrote:
This comes from an assumption in 1c12c4cf9411eb130b245fa8d0fbbaf989477c7b
mprotect: prevent alteration of the PAT bits, that PTE_MASK is what it's
supposed to be: whereas it's been wrong forever with PAE, staying 32-bit
where 64-bit is needed.

Can we *please* just fix PTE_MASK?

And can we agree to never EVER use that PAGE_MASK thing (which was only ever meant to work on *addresses*) for any pte operations (including the definition of PTE_MASK)? Because PAGE_MASK is very much the word-size, and in 32-bit PAE, the page table entry is bigger.

IOE, PTE_MASK should be a "pteval_t". And it should have absolutely *nothing* to do with PAGE_MASK. EVER.

IOW, maybe something like this?

That's pretty close to the core of my patches (just reposted), which have been cooking in x86.git for a week or so.

One thing I'd take from your patch is something like your __PHYSICAL_LOW_BITS definition, since its a bit clearer than what I did. (I haven't updated my patch before posting just because I wanted to post exactly as tested.)

And no, I haven't tested this at all. But it should make PTE_MASK have
(a) the right type ("pteval_t", not "long" - the latter is pure and utter crap)
(b) the right value (proper mask, not a sign-extended long - again, the latter is pure and utter crap)

but for all I know there might be some broken code that depends on the current incorrect and totally broken #defines, so this needs testing and thinking about.

It also causes these warnings on 32-bit PAE:

AS arch/x86/kernel/head_32.o
arch/x86/kernel/head_32.S: Assembler messages:
arch/x86/kernel/head_32.S:225: Warning: left operand is a bignum; integer 0 assumed
arch/x86/kernel/head_32.S:609: Warning: left operand is a bignum; integer 0 assumed

and I do not see why (the end result seems to be identical).

Ingo, comments?

Oh, and those #define's should be moved from <asm/page.h> to <asm/pgtable.h>, I think. They have nothing to do with pages (despite the name of "physical_page_mask", and really are meaningful only in the context of some kind of page table entry.


include/asm-x86/page.h | 5 +++--
1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/asm-x86/page.h b/include/asm-x86/page.h
index b381f4a..34b4845 100644
--- a/include/asm-x86/page.h
+++ b/include/asm-x86/page.h
@@ -10,8 +10,8 @@
#ifdef __KERNEL__
+#define PTE_MASK (_AT(pteval_t, PHYSICAL_PAGE_MASK))
#define PMD_PAGE_SIZE (_AC(1, UL) << PMD_SHIFT)
@@ -24,6 +24,7 @@
/* to align the pointer to the (next) page boundary */
#define PAGE_ALIGN(addr) (((addr)+PAGE_SIZE-1)&PAGE_MASK)
+#define __PHYSICAL_LOW_BITS _AT(phys_addr_t, (PAGE_SIZE-1))
#define __PHYSICAL_MASK _AT(phys_addr_t, (_AC(1,ULL) << __PHYSICAL_MASK_SHIFT) - 1)
#define __VIRTUAL_MASK ((_AC(1,UL) << __VIRTUAL_MASK_SHIFT) - 1)

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