* Re: x86_64 expertise needed re: BUG() at pageattr:107
2004-11-01 20:59 x86_64 expertise needed re: BUG() at pageattr:107 John W. Linville
@ 2004-11-02 19:02 ` Jason Baron
0 siblings, 0 replies; 2+ messages in thread
From: Jason Baron @ 2004-11-02 19:02 UTC (permalink / raw)
To: John W. Linville; +Cc: linux-kernel
On Mon, 1 Nov 2004, John W. Linville wrote:
> The x86_64 version of pageattr.c contains the following definition
> of revert_page():
>
> static void revert_page(unsigned long address, pgprot_t ref_prot)
> {
> pgd_t *pgd;
> pmd_t *pmd;
> pte_t large_pte;
>
> pgd = pgd_offset_k(address);
> pmd = pmd_offset(pgd, address);
> BUG_ON(pmd_val(*pmd) & _PAGE_PSE);
> pgprot_val(ref_prot) |= _PAGE_PSE;
> large_pte = mk_pte_phys(__pa(address) & LARGE_PAGE_MASK, ref_prot);
> set_pte((pte_t *)pmd, large_pte);
> }
>
> Please notice the BUG_ON() (originally at line pageattr.c:107).
> I notice that the x86 version of revert_page() has no similar check.
>
> Would someone please explain:
>
> a) why is this a bug?;
In this code path, a pmd is 'reverted' to point to a 2MB/4MB span of
physical memory. This memory had previously been split from a 2MB/4MB
span into 4KB segments, adding a pte table in the process, by calling
change_page_attr. Thus, when it is reverted back it should not be pointing
to the 2MB/4MB sapn.
> b) what is likely to have triggered it?; and,
not sure, but i think when the page splits the count should be increased
in the pte page, not the pmd page. as follows:
--- linux/arch/i386/mm/pageattr.c.orig Tue Nov 2 13:48:39 2004
+++ linux/arch/i386/mm/pageattr.c Tue Nov 2 13:49:02 2004
@@ -126,7 +126,7 @@ __change_page_attr(struct page *page, pg
struct page *split = split_large_page(address, prot);
if (!split)
return -ENOMEM;
- get_page(kpte_page);
+ get_page(split);
set_pmd_pte(kpte,address,mk_pte(split, PAGE_KERNEL));
}
} else if ((pte_val(*kpte) & _PAGE_PSE) == 0) {
--- 2.6-10-28-04/arch/x86_64/mm/pageattr.c.orig Tue Nov 2 13:41:34 2004
+++ 2.6-10-28-04/arch/x86_64/mm/pageattr.c Tue Nov 2 13:48:28 2004
@@ -134,7 +134,7 @@ __change_page_attr(unsigned long address
struct page *split = split_large_page(address, prot, ref_prot);
if (!split)
return -ENOMEM;
- get_page(kpte_page);
+ get_page(split);
set_pte(kpte,mk_pte(split, ref_prot));
}
} else if ((kpte_flags & _PAGE_PSE) == 0) {
> c) why is x86_64 different from x86?
This should be a BUG condition on x86 as well.
-Jason
^ permalink raw reply [flat|nested] 2+ messages in thread