[PATCH] mm, proc: Make the task_mmu walk_page_range() limit in clear_refs_write() obvious

[PATCH] mm, proc: Make the task_mmu walk_page_range() limit in clear_refs_write() obvious

[James Morse]

Trying to walk all of virtual memory requires architecture specific
knowledge. On x86_64, addresses must be sign extended from bit 48,
whereas on arm64 the top VA_BITS of address space have their own set
of page tables.

clear_refs_write() calls walk_page_range() on the range 0 to ~0UL, it
provides a test_walk() callback that only expects to be walking over
VMAs. Currently walk_pmd_range() will skip memory regions that don't
have a VMA, reporting them as a hole.

As this call only expects to walk user address space, make it walk
0 to  'highest_vm_end'.

Signed-off-by: James Morse <james.morse@arm.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

---
This is in preparation for a RFC series that allows walk_page_range() to
walk kernel page tables too.

 fs/proc/task_mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 187d84ef9de9..1026b7862896 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1068,7 +1068,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 			}
 			mmu_notifier_invalidate_range_start(mm, 0, -1);
 		}
-		walk_page_range(0, ~0UL, &clear_refs_walk);
+		walk_page_range(0, mm->highest_vm_end, &clear_refs_walk);
 		if (type == CLEAR_REFS_SOFT_DIRTY)
 			mmu_notifier_invalidate_range_end(mm, 0, -1);
 		flush_tlb_mm(mm);
-- 
2.8.0.rc3

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Re: [PATCH] mm, proc: Make the task_mmu walk_page_range() limit in clear_refs_write() obvious

[Naoya Horiguchi]

On Wed, Aug 31, 2016 at 04:03:12PM +0100, James Morse wrote:
> Trying to walk all of virtual memory requires architecture specific
> knowledge. On x86_64, addresses must be sign extended from bit 48,
> whereas on arm64 the top VA_BITS of address space have their own set
> of page tables.
> 
> clear_refs_write() calls walk_page_range() on the range 0 to ~0UL, it
> provides a test_walk() callback that only expects to be walking over
> VMAs. Currently walk_pmd_range() will skip memory regions that don't
> have a VMA, reporting them as a hole.
> 
> As this call only expects to walk user address space, make it walk
> 0 to  'highest_vm_end'.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Makes sense to me.

Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
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