Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding

From: Christophe Leroy
Date: Tue Sep 08 2020 - 01:06:48 EST

Le 07/09/2020 à 20:00, Gerald Schaefer a écrit :
From: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>

Commit 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast
code") introduced a subtle but severe bug on s390 with gup_fast, due to
dynamic page table folding.

The question "What would it require for the generic code to work for s390"
has already been discussed here
and ended with a promising approach here
which in the end unfortunately didn't quite work completely.

We tried to mimic static level folding by changing pgd_offset to always
calculate top level page table offset, and do nothing in folded pXd_offset.
What has been overlooked is that PxD_SIZE/MASK and thus pXd_addr_end do
not reflect this dynamic behaviour, and still act like static 5-level
page tables.


Fix this by introducing new pXd_addr_end_folded helpers, which take an
additional pXd entry value parameter, that can be used on s390
to determine the correct page table level and return corresponding
end / boundary. With that, the pointer iteration will always
happen in gup_pgd_range for s390. No change for other architectures

Not sure pXd_addr_end_folded() is the best understandable name, allthough I don't have any alternative suggestion at the moment.
Maybe could be something like pXd_addr_end_fixup() as it will disappear in the next patch, or pXd_addr_end_gup() ?

Also, if it happens to be acceptable to get patch 2 in stable, I think you should switch patch 1 and patch 2 to avoid the step through pXd_addr_end_folded()

Fixes: 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.2+
Reviewed-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
arch/s390/include/asm/pgtable.h | 42 +++++++++++++++++++++++++++++++++
include/linux/pgtable.h | 16 +++++++++++++
mm/gup.c | 8 +++----
3 files changed, 62 insertions(+), 4 deletions(-)

diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
index 7eb01a5459cd..027206e4959d 100644
--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -512,6 +512,48 @@ static inline bool mm_pmd_folded(struct mm_struct *mm)
#define mm_pmd_folded(mm) mm_pmd_folded(mm)
+ * With dynamic page table levels on s390, the static pXd_addr_end() functions
+ * will not return corresponding dynamic boundaries. This is no problem as long
+ * as only pXd pointers are passed down during page table walk, because
+ * pXd_offset() will simply return the given pointer for folded levels, and the
+ * pointer iteration over a range simply happens at the correct page table
+ * level.
+ * It is however a problem with gup_fast, or other places walking the page
+ * tables w/o locks using READ_ONCE(), and passing down the pXd values instead
+ * of pointers. In this case, the pointer given to pXd_offset() is a pointer to
+ * a stack variable, which cannot be used for pointer iteration at the correct
+ * level. Instead, the iteration then has to happen by going up to pgd level
+ * again. To allow this, provide pXd_addr_end_folded() functions with an
+ * additional pXd value parameter, which can be used on s390 to determine the
+ * folding level and return the corresponding boundary.
+ */
+static inline unsigned long rste_addr_end_folded(unsigned long rste, unsigned long addr, unsigned long end)

What does 'rste' stands for ?

Isn't this line a bit long ?

+ unsigned long type = (rste & _REGION_ENTRY_TYPE_MASK) >> 2;
+ unsigned long size = 1UL << (_SEGMENT_SHIFT + type * 11);
+ unsigned long boundary = (addr + size) & ~(size - 1);
+ /*
+ * FIXME The below check is for internal testing only, to be removed
+ */
+ VM_BUG_ON(type < (_REGION_ENTRY_TYPE_R3 >> 2));
+ return (boundary - 1) < (end - 1) ? boundary : end;
+#define pgd_addr_end_folded pgd_addr_end_folded
+static inline unsigned long pgd_addr_end_folded(pgd_t pgd, unsigned long addr, unsigned long end)
+ return rste_addr_end_folded(pgd_val(pgd), addr, end);
+#define p4d_addr_end_folded p4d_addr_end_folded
+static inline unsigned long p4d_addr_end_folded(p4d_t p4d, unsigned long addr, unsigned long end)
+ return rste_addr_end_folded(p4d_val(p4d), addr, end);
static inline int mm_has_pgste(struct mm_struct *mm)
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index e8cbc2e795d5..981c4c2a31fe 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -681,6 +681,22 @@ static inline int arch_unmap_one(struct mm_struct *mm,
+#ifndef pgd_addr_end_folded
+#define pgd_addr_end_folded(pgd, addr, end) pgd_addr_end(addr, end)
+#ifndef p4d_addr_end_folded
+#define p4d_addr_end_folded(p4d, addr, end) p4d_addr_end(addr, end)
+#ifndef pud_addr_end_folded
+#define pud_addr_end_folded(pud, addr, end) pud_addr_end(addr, end)
+#ifndef pmd_addr_end_folded
+#define pmd_addr_end_folded(pmd, addr, end) pmd_addr_end(addr, end)
* When walking page tables, we usually want to skip any p?d_none entries;
* and any p?d_bad entries - reporting the error before resetting to none.
diff --git a/mm/gup.c b/mm/gup.c
index bd883a112724..ba4aace5d0f4 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2521,7 +2521,7 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
do {
pmd_t pmd = READ_ONCE(*pmdp);
- next = pmd_addr_end(addr, end);
+ next = pmd_addr_end_folded(pmd, addr, end);
if (!pmd_present(pmd))
return 0;
@@ -2564,7 +2564,7 @@ static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
do {
pud_t pud = READ_ONCE(*pudp);
- next = pud_addr_end(addr, end);
+ next = pud_addr_end_folded(pud, addr, end);
if (unlikely(!pud_present(pud)))
return 0;
if (unlikely(pud_huge(pud))) {
@@ -2592,7 +2592,7 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end,
do {
p4d_t p4d = READ_ONCE(*p4dp);
- next = p4d_addr_end(addr, end);
+ next = p4d_addr_end_folded(p4d, addr, end);
if (p4d_none(p4d))
return 0;
@@ -2617,7 +2617,7 @@ static void gup_pgd_range(unsigned long addr, unsigned long end,
do {
pgd_t pgd = READ_ONCE(*pgdp);
- next = pgd_addr_end(addr, end);
+ next = pgd_addr_end_folded(pgd, addr, end);
if (pgd_none(pgd))
if (unlikely(pgd_huge(pgd))) {