On 07/14/2015 06:04 PM, Catalin Marinas wrote:

On Fri, Jul 10, 2015 at 08:11:03PM +0300, Andrey Ryabinin wrote:

quoted

quoted

quoted

+#if CONFIG_PGTABLE_LEVELS > 3
+pud_t kasan_zero_pud[PTRS_PER_PUD] __page_aligned_bss;
+#endif
+#if CONFIG_PGTABLE_LEVELS > 2
+pmd_t kasan_zero_pmd[PTRS_PER_PMD] __page_aligned_bss;
+#endif
+pte_t kasan_zero_pte[PTRS_PER_PTE] __page_aligned_bss;
+
+static void __init kasan_early_pmd_populate(unsigned long start,
+					unsigned long end, pud_t *pud)
+{
+	unsigned long addr;
+	unsigned long next;
+	pmd_t *pmd;
+
+	pmd = pmd_offset(pud, start);
+	for (addr = start; addr < end; addr = next, pmd++) {
+		pmd_populate_kernel(&init_mm, pmd, kasan_zero_pte);
+		next = pmd_addr_end(addr, end);
+	}
+}
+
+static void __init kasan_early_pud_populate(unsigned long start,
+					unsigned long end, pgd_t *pgd)
+{
+	unsigned long addr;
+	unsigned long next;
+	pud_t *pud;
+
+	pud = pud_offset(pgd, start);
+	for (addr = start; addr < end; addr = next, pud++) {
+		pud_populate(&init_mm, pud, kasan_zero_pmd);
+		next = pud_addr_end(addr, end);
+		kasan_early_pmd_populate(addr, next, pud);
+	}
+}
+
+static void __init kasan_map_early_shadow(pgd_t *pgdp)
+{
+	int i;
+	unsigned long start = KASAN_SHADOW_START;
+	unsigned long end = KASAN_SHADOW_END;
+	unsigned long addr;
+	unsigned long next;
+	pgd_t *pgd;
+
+	for (i = 0; i < PTRS_PER_PTE; i++)
+		set_pte(&kasan_zero_pte[i], pfn_pte(
+				virt_to_pfn(kasan_zero_page), PAGE_KERNEL));
+
+	pgd = pgd_offset_k(start);
+	for (addr = start; addr < end; addr = next, pgd++) {
+		pgd_populate(&init_mm, pgd, kasan_zero_pud);
+		next = pgd_addr_end(addr, end);
+		kasan_early_pud_populate(addr, next, pgd);
+	}

I prefer to use "do ... while" constructs similar to __create_mapping()
(or zero_{pgd,pud,pmd}_populate as you are more familiar with them).

But what I don't get here is that you repopulate the pud page for every
pgd (and so on for pmd). You don't need this recursive call all the way
to kasan_early_pmd_populate() but just sequential:

This repopulation needed for 3,2 level page tables configurations.

E.g. for 3-level page tables we need to call pud_populate(&init_mm,
pud, kasan_zero_pmd) for each pud in [KASAN_SHADOW_START,
KASAN_SHADOW_END] range, this causes repopopulation for 4-level page
tables, since we need to pud_populate() only [KASAN_SHADOW_START,
KASAN_SHADOW_START + PGDIR_SIZE] range.

I'm referring to writing the same information multiple times over the
same entry. kasan_map_early_shadow() goes over each pgd entry and writes
the address of kasan_zero_pud. That's fine so far. However, in the same
loop you call kasan_early_pud_populate(). The latter retrieves the pud
page via pud_offset(pgd, start) which would always be kasan_zero_pud

Not always. E.g. if we have 3-level page tables pud = pgd, pgd_populate() is nop, and
pud_populate in fact populates pgd.

pud_offset(pgd, start) will return (swapper_pg_dir + pgd_index(start)) and pud_populate()
will fill that entry with the address of kasan_zero_pmd. So we need to pud_populate() for
each pgd.

because that's what you wrote via pgd_populate() in each pgd entry. So
for each pgd entry, you keep populating the same kasan_zero_pud page
with pointers to kasan_zero_pmd. And so on for the pmd.

Yes, I'm perfectly understand that. And this was done intentionally since I don't
see the way to make this work for all possible CONFIG_PGTABLE_LEVELS without rewrites
or without #ifdefs (and you didn't like them in v1).

quoted

quoted

	kasan_early_pte_populate();
	kasan_early_pmd_populate(..., pte);
	kasan_early_pud_populate(..., pmd);
	kasan_early_pgd_populate(..., pud);

(or in reverse order)

Unless, I'm missing something, this will either work only with 4-level
page tables. We could do this without repopulation by using
CONFIG_PGTABLE_LEVELS ifdefs.

Or you could move kasan_early_*_populate outside the loop. You already
do this for the pte at the beginning of the kasan_map_early_shadow()
function (and it probably makes more sense to create a separate
kasan_early_pte_populate).

Ok, let's try to implement that.
And for example, let's consider CONFIG_PGTABLE_LEVELS=3 case:

 * pgd_populate() is nop, so kasan_early_pgd_populate() won't do anything.

 * pud_populate() in kasan_early_pud_populate() actually will setup pgd entries in swapper_pg_dir,
   so pud_populate() should be called for the whole shadow range: [KASAN_SHADOW_START, KASAN_SHADOW_END]
	IOW: kasan_early_pud_populate(KASAN_SHADOW_START, KASAN_SHADOW_END, kasan_zero_pmd);
	
	We will need to slightly change kasan_early_pud_populate() implementation for that
	(Current implementation implies that [start, end) addresses belong to one pgd)

	void kasan_early_pud_populate(unsigned long start, unsigned long end, pmd_t *pmd)
	{
		unsigned long addr;
		long next;

		for (addr = start; addr < end; addr = next) {
			pud_t *pud = pud_offset(pgd_offset_k(addr), addr);
			pud_populate(&init_mm, pud, pmd);
			next = pud_addr_end(addr, pgd_addr_end(addr, end));
		}
	}

	But, wait! In 4-level page tables case this will be the same repopulation as we had before!

See? The problem here is that pud_populate() but not pgd_populate() populates pgds (3-level page tables case).
So I still don't see the way to avoid repopulation without ifdefs.
Did I miss anything?