Re: [PATCH v3 1/7] mm: Add functions to track page directory modifications

Catalin Marinas
2020-06-05T10:08:21Z

Hi Joerg,

On Fri, May 15, 2020 at 04:00:17PM +0200, Joerg Roedel wrote:
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5a323422d783..022fe682af9e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2078,13 +2078,54 @@ static inline pud_t *pud_alloc(struct mm_struct *mm, p4d_t *p4d,
> return (unlikely(p4d_none(*p4d)) && __pud_alloc(mm, p4d, address)) ?
> NULL : pud_offset(p4d, address);
> }
> +
> +static inline p4d_t *p4d_alloc_track(struct mm_struct *mm, pgd_t *pgd,
> + unsigned long address,
> + pgtbl_mod_mask *mod_mask)
> +
> +{
> + if (unlikely(pgd_none(*pgd))) {
> + if (__p4d_alloc(mm, pgd, address))
> + return NULL;
> + *mod_mask |= PGTBL_PGD_MODIFIED;
> + }
> +
> + return p4d_offset(pgd, address);
> +}
> +
> #endif /* !__ARCH_HAS_5LEVEL_HACK */
>
> +static inline pud_t *pud_alloc_track(struct mm_struct *mm, p4d_t *p4d,
> + unsigned long address,
> + pgtbl_mod_mask *mod_mask)
> +{
> + if (unlikely(p4d_none(*p4d))) {
> + if (__pud_alloc(mm, p4d, address))
> + return NULL;
> + *mod_mask |= PGTBL_P4D_MODIFIED;
> + }
> +
> + return pud_offset(p4d, address);
> +}

This patch causes a kernel panic on arm64 (and possibly powerpc, I
haven't tried). arm64 still uses the 5level-fixup.h and pud_alloc()
checks for the empty p4d with pgd_none() instead of p4d_none().

The patch below fixes it:

-----------------------8<-----------------------------------
From 8fb5f4c1e0983bb501c8ec71f9bb9dd344b80e87 Mon Sep 17 00:00:00 2001
From: Catalin Marinas <catalin.marinas@arm.com>
Date: Fri, 5 Jun 2020 10:56:03 +0100
Subject: [PATCH] mm: Use pgd_none() in pud_alloc_track() with
__ARCH_HAS_5LEVEL_HACK

Commit d8626138009b ("mm: add functions to track page directory
modifications") introduced the pud_alloc_track() function checking for
an empty p4d using p4d_none(). However, when __ARCH_HAS_5LEVEL_HACK is
defined, the pud_alloc() counterpart checks for an empty p4d using
pgd_none(). Since p4d_none() is always 0 in this case, no pud would be
allocated and the kernel panics during boot on arm64 (at least).

Until all architectures are moved away from the 5level-fixup.h, define a
pud_alloc_track() that matches the __ARCH_HAS_5LEVEL_HACK pud_alloc().

Fixes: d8626138009b ("mm: add functions to track page directory modifications")
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
include/linux/mm.h | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index fda41eb7f1c8..9d3761a1fad5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2106,8 +2106,6 @@ static inline p4d_t *p4d_alloc_track(struct mm_struct *mm, pgd_t *pgd,
return p4d_offset(pgd, address);
}

-#endif /* !__ARCH_HAS_5LEVEL_HACK */
-
static inline pud_t *pud_alloc_track(struct mm_struct *mm, p4d_t *p4d,
unsigned long address,
pgtbl_mod_mask *mod_mask)
@@ -2121,6 +2119,23 @@ static inline pud_t *pud_alloc_track(struct mm_struct *mm, p4d_t *p4d,
return pud_offset(p4d, address);
}

+#else /* __ARCH_HAS_5LEVEL_HACK */
+
+static inline pud_t *pud_alloc_track(struct mm_struct *mm, p4d_t *p4d,
+ unsigned long address,
+ pgtbl_mod_mask *mod_mask)
+{
+ if (unlikely(pgd_none(*p4d))) {
+ if (__pud_alloc(mm, p4d, address))
+ return NULL;
+ *mod_mask |= PGTBL_P4D_MODIFIED;
+ }
+
+ return pud_offset(p4d, address);
+}
+
+#endif /* !__ARCH_HAS_5LEVEL_HACK */
+
static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address)
{
return (unlikely(pud_none(*pud)) && __pmd_alloc(mm, pud, address))?