Re: [RFC PATCH v2 2/3] mm: make pXd_addr_end() functions page-table entry aware

Alexander Gordeev
2020-09-08T14:15:55Z

On Tue, Sep 08, 2020 at 10:16:49AM +0200, Christophe Leroy wrote:
> >Yes, and also two more sources :/
> > arch/powerpc/mm/kasan/8xx.c
> > arch/powerpc/mm/kasan/kasan_init_32.c
> >
> >But these two are not quite obvious wrt pgd_addr_end() used
> >while traversing pmds. Could you please clarify a bit?
> >
> >
> >diff --git a/arch/powerpc/mm/kasan/8xx.c b/arch/powerpc/mm/kasan/8xx.c
> >index 2784224..89c5053 100644
> >--- a/arch/powerpc/mm/kasan/8xx.c
> >+++ b/arch/powerpc/mm/kasan/8xx.c
> >@@ -15,8 +15,8 @@
> > for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pmd += 2, block += SZ_8M) {
> > pte_basic_t *new;
> >- k_next = pgd_addr_end(k_cur, k_end);
> >- k_next = pgd_addr_end(k_next, k_end);
> >+ k_next = pmd_addr_end(k_cur, k_end);
> >+ k_next = pmd_addr_end(k_next, k_end);
>
> No, I don't think so.
> On powerpc32 we have only two levels, so pgd and pmd are more or
> less the same.
> But pmd_addr_end() as defined in include/asm-generic/pgtable-nopmd.h
> is a no-op, so I don't think it will work.
>
> It is likely that this function should iterate on pgd, then you get
> pmd = pmd_offset(pud_offset(p4d_offset(pgd)));

It looks like the code iterates over single pmd table while using
pgd_addr_end() only to skip all the middle levels and bail out
from the loop.

I would be wary for switching from pmds to pgds, since we are
trying to minimize impact (especially functional) and the
rework does not seem that obvious.

Assuming pmd and pgd are the same would actually such approach
work for now?

diff --git a/arch/powerpc/mm/kasan/8xx.c b/arch/powerpc/mm/kasan/8xx.c
index 2784224..94466cc 100644
--- a/arch/powerpc/mm/kasan/8xx.c
+++ b/arch/powerpc/mm/kasan/8xx.c
@@ -15,8 +15,8 @@
for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pmd += 2, block += SZ_8M) {
pte_basic_t *new;

- k_next = pgd_addr_end(k_cur, k_end);
- k_next = pgd_addr_end(k_next, k_end);
+ k_next = pgd_addr_end(__pgd(pmd_val(*pmd)), k_cur, k_end);
+ k_next = pgd_addr_end(__pgd(pmd_val(*(pmd + 1))), k_next, k_end);
if ((void *)pmd_page_vaddr(*pmd) != kasan_early_shadow_pte)
continue;

diff --git a/arch/powerpc/mm/kasan/kasan_init_32.c b/arch/powerpc/mm/kasan/kasan_init_32.c
index fb29404..c0bcd64 100644
--- a/arch/powerpc/mm/kasan/kasan_init_32.c
+++ b/arch/powerpc/mm/kasan/kasan_init_32.c
@@ -38,7 +38,7 @@ int __init kasan_init_shadow_page_tables(unsigned long k_start, unsigned long k_
for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pmd++) {
pte_t *new;

- k_next = pgd_addr_end(k_cur, k_end);
+ k_next = pgd_addr_end(__pgd(pmd_val(*pmd)), k_cur, k_end);
if ((void *)pmd_page_vaddr(*pmd) != kasan_early_shadow_pte)
continue;

@@ -196,7 +196,7 @@ void __init kasan_early_init(void)
kasan_populate_pte(kasan_early_shadow_pte, PAGE_KERNEL);

do {
- next = pgd_addr_end(addr, end);
+ next = pgd_addr_end(__pgd(pmd_val(*pmd)), addr, end);
pmd_populate_kernel(&init_mm, pmd, kasan_early_shadow_pte);
} while (pmd++, addr = next, addr != end);

Alternatively we could pass invalid pgd to keep the code structure
intact, but that of course is less nice.

Thanks!

> Christophe