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SubjectRe: [PATCH] fix/improve generic page table walker
On Thu, 12 Mar 2009 10:58:14 -0500
Matt Mackall <mpm@selenic.com> wrote:

> On Thu, 2009-03-12 at 15:42 +0100, Martin Schwidefsky wrote:
> > Then what exactly is a pgd_t? For me it is the top level page table
> > which can have very different meaning for the various architectures.
>
> The important thing is that it's always 3 levels removed from the
> bottom, whether or not those 3 levels actually have hardware
> manifestations. From your description, it sounds like that's not how
> things work in S390 land.

With the page table folding "3 levels removed from the bottom" doesn't
tell me much since there is no real representation in hardware AND in
memory for the missing page table levels. So the only valid meaning of
a pgd_t is that you have to use pud_offset, pmd_offset and pte_offset
to get to a pte. If I do the page table folding at runtime or at
compile time is a minor detail.

> > Well, the hardware can do up to 5 levels of page tables for the full
> > 64 bit address space. With the introduction of pud's we wanted to
> > extend our address space from 3 levels / 42 bits to 4 levels / 53 bits.
> > But this comes at a cost: additional page table levels cost memory and
> > performance. In particular for the compat processes which can only
> > address a maximum of 2 GB it is a waste to allocate 4 levels. With the
> > dynamic page tables we allocate as much as required by each process.
>
> X86 uses 1-entry tables at higher levels to maintain consistency with
> fairly minimal overhead. In some of the sillier addressing modes, we may
> even use a 4-entry table in some places. I think table size is fixed at
> compile time, but I don't think that's essential. Very little code in
> the x86 architecture has any notion of how many hardware levels actually
> exist.

Indeed very little code needs to know how many page table levels
exist. The page table folding works as long as the access to a
particular page is done with the sequence

pgd = pgd_offset(mm, address);
pud = pud_offset(pgd, address);
pmd = pmd_offset(pud, address);
pte = pte_offset(pmd, address);

The indivitual pointers pgd/pud/pmd/pte can be incremented as long as
they stay in the valid address range, e.g. pmd_addr_end checks for the
next pmd segment boundary and the end address of the walk.

If the page table folding is static or dynamic is irrelevant. The only
thing we are arguing is what makes a valid end address for a walk. It
has to be smaller than TASK_SIZE. With the current definitions the s390
code has the additional assumption that the address has to be smaller
than the highest vma as well. The patch below changes TASK_SIZE to
reflect the size of the address space in use by the process. Then the
generic page table walker works fine. What doesn't work anymore is the
automatic upgrade from 3 to 4 levels via mmap. I'm still thinking about
a clever solution, the best I have so far is a patch that introduces
TASK_SIZE_MAX which reflects the maximum possible size as opposed to
TASK_SIZE that gives you the current size. The code in do_mmap_pgoff
then uses TASK_SIZE_MAX instead of TASK_SIZE.

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.

---
Subject: [PATCH] make page table walking more robust

From: Martin Schwidefsky <schwidefsky@de.ibm.com>

Make page table walking on s390 more robust. The current code requires
that the pgd/pud/pmd/pte loop is only done for address ranges that are
below the end address of the last vma of the address space. But this
is not always true, e.g. the generic page table walker does not
guarantee this. Change TASK_SIZE/TASK_SIZE_OF to reflect the current
size of the address space. This makes the generic page table walker
happy but it breaks the upgrade of a 3 level page table to a 4 level
page table. To make the upgrade work again another fix is required.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---

arch/s390/include/asm/processor.h | 5 ++---
arch/s390/mm/mmap.c | 4 ++--
arch/s390/mm/pgtable.c | 2 ++
3 files changed, 6 insertions(+), 5 deletions(-)

diff -urpN linux-2.6/arch/s390/include/asm/processor.h
linux-2.6-patched/arch/s390/include/asm/processor.h ---
linux-2.6/arch/s390/include/asm/processor.h 2009-03-16
12:24:26.000000000 +0100 +++
linux-2.6-patched/arch/s390/include/asm/processor.h 2009-03-16
12:24:28.000000000 +0100 @@ -47,7 +47,7 @@ extern void
print_cpu_info(void); extern int get_cpu_capability(unsigned int *); /*
- * User space process size: 2GB for 31 bit, 4TB for 64 bit.
+ * User space process size: 2GB for 31 bit, 4TB or 8PT for 64 bit.
*/
#ifndef __s390x__

@@ -56,8 +56,7 @@ extern int get_cpu_capability(unsigned i

#else /* __s390x__ */

-#define TASK_SIZE_OF(tsk)
(test_tsk_thread_flag(tsk,TIF_31BIT) ? \
- (1UL << 31) : (1UL << 53))
+#define TASK_SIZE_OF(tsk) ((tsk)->mm->context.asce_limit)
#define TASK_UNMAPPED_BASE (test_thread_flag(TIF_31BIT) ? \
(1UL << 30) : (1UL << 41))
#define TASK_SIZE TASK_SIZE_OF(current)
diff -urpN linux-2.6/arch/s390/mm/mmap.c
linux-2.6-patched/arch/s390/mm/mmap.c ---
linux-2.6/arch/s390/mm/mmap.c 2008-12-25 00:26:37.000000000
+0100 +++ linux-2.6-patched/arch/s390/mm/mmap.c 2009-03-16
12:24:28.000000000 +0100 @@ -35,7 +35,7 @@
* Leave an at least ~128 MB hole.
*/
#define MIN_GAP (128*1024*1024)
-#define MAX_GAP (TASK_SIZE/6*5)
+#define MAX_GAP (STACK_TOP/6*5)

static inline unsigned long mmap_base(void)
{
@@ -46,7 +46,7 @@ static inline unsigned long mmap_base(vo
else if (gap > MAX_GAP)
gap = MAX_GAP;

- return TASK_SIZE - (gap & PAGE_MASK);
+ return STACK_TOP - (gap & PAGE_MASK);
}

static inline int mmap_is_legacy(void)
diff -urpN linux-2.6/arch/s390/mm/pgtable.c
linux-2.6-patched/arch/s390/mm/pgtable.c ---
linux-2.6/arch/s390/mm/pgtable.c 2009-03-16 12:24:09.000000000
+0100 +++ linux-2.6-patched/arch/s390/mm/pgtable.c 2009-03-16
12:24:28.000000000 +0100 @@ -117,6 +117,7 @@ repeat:
crst_table_init(table, entry); pgd_populate(mm, (pgd_t *) table, (pud_t
*) pgd); mm->pgd = (pgd_t *) table;
+ mm->task_size = mm->context.asce_limit;
table = NULL;
}
spin_unlock(&mm->page_table_lock);
@@ -154,6 +155,7 @@ void crst_table_downgrade(struct mm_stru
BUG();
}
mm->pgd = (pgd_t *) (pgd_val(*pgd) &
_REGION_ENTRY_ORIGIN);
+ mm->task_size = mm->context.asce_limit;
crst_table_free(mm, (unsigned long *) pgd);
}
update_mm(mm, current);

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