[PATCH] mm: make unmap_vmas() handle non-page-aligned boundary addresses

[Johannes Weiner]

zap_pte_range() overruns the page tables if the distance between the
start and end is not a multiple of the pagesize.  Because then,
`start' will never be equal to `end' and we will keep looping.

To fix this, round the boundary addresses to exclude partial pages from
the range completely, we must not unmap them anyway.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxxx>
---

Hugh Dickins <hugh@xxxxxxxxxxx> writes:

> On Sat, 16 Aug 2008, Rafael J. Wysocki wrote:
>> 
>> Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=11335
>> Subject		: 2.6.27-rc2-git5 BUG: unable to handle kernel paging request
>> Submitter	: Randy Dunlap <randy.dunlap@xxxxxxxxxx>
>> Date		: 2008-08-12 4:18 (5 days old)
>> References	: http://marc.info/?l=linux-kernel&m=121851477201960&w=4
>> Handled-By	: Hugh Dickins <hugh@xxxxxxxxxxx>
>
> This should still be listed for now, it's interesting,
> but I doubt we'll make any progress unless it can be reproduced.

I think this patch fixes it.  exit_mmap() even calls unmap_vmas() with
an ending address of -1UL which is not page-aligned in my book and on my
architecture :)

It is a similar problem to what we had with gup some weeks ago.

diff --git a/mm/memory.c b/mm/memory.c
index 1002f47..483c5d0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -896,11 +896,17 @@ unsigned long unmap_vmas(struct mmu_gather **tlbp,
 	long zap_work = ZAP_BLOCK_SIZE;
 	unsigned long tlb_start = 0;	/* For tlb_finish_mmu */
 	int tlb_start_valid = 0;
-	unsigned long start = start_addr;
+	unsigned long start;
 	spinlock_t *i_mmap_lock = details? details->i_mmap_lock: NULL;
 	int fullmm = (*tlbp)->fullmm;
 	struct mm_struct *mm = vma->vm_mm;
 
+	/* Preserve partial pages */
+	start_addr = PAGE_ALIGN(start_addr);
+	end_addr &= PAGE_MASK;
+
+	start = start_addr;
+
 	mmu_notifier_invalidate_range_start(mm, start_addr, end_addr);
 	for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next) {
 		unsigned long end;
--
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