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Subject: Re: [RFC PATCH v2 2/3] mm: make pXd_addr_end() functions
 page-table entry aware
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
To:     Alexander Gordeev <agordeev@linux.ibm.com>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
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Organization: CS Group France
Date:   Wed, 09 Sep 2020 08:38:31 +0000
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On Tue, 2020-09-08 at 16:15 +0200, Alexander Gordeev wrote:
> 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.
> 

I've just tested the following change, it works and should fix the
oddity:

diff --git a/arch/powerpc/mm/kasan/8xx.c b/arch/powerpc/mm/kasan/8xx.c
index 2784224054f8..8e53ddf57b84 100644
--- a/arch/powerpc/mm/kasan/8xx.c
+++ b/arch/powerpc/mm/kasan/8xx.c
@@ -9,11 +9,12 @@
 static int __init
 kasan_init_shadow_8M(unsigned long k_start, unsigned long k_end, void
*block)
 {
-	pmd_t *pmd = pmd_off_k(k_start);
+	pgd_t *pgd = pgd_offset_k(k_start);
 	unsigned long k_cur, k_next;
 
-	for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pmd += 2, block
+= SZ_8M) {
+	for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pgd += 2, block
+= SZ_8M) {
 		pte_basic_t *new;
+		pmd_t *pmd = pmd_offset(pud_offset(p4d_offset(pgd, k_cur), k_cur),
k_cur);
 
 		k_next = pgd_addr_end(k_cur, k_end);
 		k_next = pgd_addr_end(k_next, k_end);
diff --git a/arch/powerpc/mm/kasan/kasan_init_32.c
b/arch/powerpc/mm/kasan/kasan_init_32.c
index fb294046e00e..e5f524fa71a7 100644
--- a/arch/powerpc/mm/kasan/kasan_init_32.c
+++ b/arch/powerpc/mm/kasan/kasan_init_32.c
@@ -30,13 +30,12 @@ static void __init kasan_populate_pte(pte_t *ptep,
pgprot_t prot)
 
 int __init kasan_init_shadow_page_tables(unsigned long k_start,
unsigned long k_end)
 {
-	pmd_t *pmd;
+	pgd_t *pgd = pgd_offset_k(k_start);
 	unsigned long k_cur, k_next;
 
-	pmd = pmd_off_k(k_start);
-
-	for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pmd++) {
+	for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pgd++) {
 		pte_t *new;
+		pmd_t *pmd = pmd_offset(pud_offset(p4d_offset(pgd, k_cur), k_cur),
k_cur);
 
 		k_next = pgd_addr_end(k_cur, k_end);
 		if ((void *)pmd_page_vaddr(*pmd) != kasan_early_shadow_pte)
@@ -189,16 +188,18 @@ void __init kasan_early_init(void)
 	unsigned long addr = KASAN_SHADOW_START;
 	unsigned long end = KASAN_SHADOW_END;
 	unsigned long next;
-	pmd_t *pmd = pmd_off_k(addr);
+	pgd_t *pgd = pgd_offset_k(addr);
 
 	BUILD_BUG_ON(KASAN_SHADOW_START & ~PGDIR_MASK);
 
 	kasan_populate_pte(kasan_early_shadow_pte, PAGE_KERNEL);
 
 	do {
+		pmd_t *pmd = pmd_offset(pud_offset(p4d_offset(pgd, addr), addr),
addr);
+
 		next = pgd_addr_end(addr, end);
 		pmd_populate_kernel(&init_mm, pmd, kasan_early_shadow_pte);
-	} while (pmd++, addr = next, addr != end);
+	} while (pgd++, addr = next, addr != end);
 
 	if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE))
 		kasan_early_hash_table();
---
Christophe