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Subjectpage table page->index
I found a situation where page->index for a pagetable page can be set
to 0 instead of the correct value. This means that ptep_to_address
will return the wrong answer. The problem occurs when remap_pmd_range
calls pte_alloc_map and pte_alloc_map needs to allocate a new pte
page, because remap_pmd_range has masked off the top bits of the
address (to avoid overflow in the computation of `end'), and it passes
the masked address to pte_alloc_map.

Now we presumably don't need to get from the physical pages mapped by
remap_page_range back to the ptes mapping them. But we could easily
map some normal pages using ptes in that pagetable page subsequently,
and when we call ptep_to_address on their ptes it will give the wrong
answer.

The patch below fixes the problem.

There is a more general question this brings up - some of the
procedures which iterate over ranges of ptes will do the wrong thing
if the end of the address range is too close to ~0UL, while others are
OK. Is this a problem in practice? On i386, ppc, and the 64-bit
architectures it isn't since user addresses can't go anywhere near
~0UL, but what about arm or m68k for instance?

And BTW, being able to go from a pte pointer to the mm and virtual
address that that pte maps is an extremely useful thing on ppc, since
it will enable me to do MMU hash-table management at set_pte (and
ptep_*) time and thus avoid the extra traversal of the pagetables that
I am currently doing in flush_tlb_*. So if you do decide to back out
rmap, please leave in the hooks for setting page->mapping and
page->index on pagetable pages.

Paul.

diff -urN linux-2.5/mm/memory.c pmac-2.5/mm/memory.c
--- linux-2.5/mm/memory.c Thu Jul 18 05:27:58 2002
+++ pmac-2.5/mm/memory.c Sat Jul 27 15:42:18 2002
@@ -863,18 +863,19 @@
static inline int remap_pmd_range(struct mm_struct *mm, pmd_t * pmd, unsigned long address, unsigned long size,
unsigned long phys_addr, pgprot_t prot)
{
- unsigned long end;
+ unsigned long base, end;

+ base = address & PGDIR_MASK;
address &= ~PGDIR_MASK;
end = address + size;
if (end > PGDIR_SIZE)
end = PGDIR_SIZE;
phys_addr -= address;
do {
- pte_t * pte = pte_alloc_map(mm, pmd, address);
+ pte_t * pte = pte_alloc_map(mm, pmd, base + address);
if (!pte)
return -ENOMEM;
- remap_pte_range(pte, address, end - address, address + phys_addr, prot);
+ remap_pte_range(pte, base + address, end - address, address + phys_addr, prot);
pte_unmap(pte);
address = (address + PMD_SIZE) & PMD_MASK;
pmd++;
-
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